feat: support fork from any conversation

docs/developers/daemon/09-event-schema.md

@@ -124,16 +124,16 @@ These events are workspace-keyed, not session-keyed. The session reducer treats
 
 ### Turn lifecycle / assistant pushes
 
-| Type                  | Direction | Trigger                                                                                                             | Key payload fields                                                                                                                                                                               |
-| --------------------- | --------- | ------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
-| `prompt_cancelled`    | S->C      | Prompt was cancelled through explicit `cancelSession` route **or** originator SSE disconnect                        | Envelope stamps `originatorClientId` for the canceling client. This means "cancellation requested", not "cancellation confirmed". Peer subscribers learn that the prompt has ended.              |
-| `turn_complete`       | S->C      | A turn completed successfully                                                                                       | `sessionId, stopReason, promptId?`. `promptId` links to non-blocking prompt responses (`202`). The SDK matches SSE events to the originating prompt through it.                                  |
-| `turn_error`          | S->C      | A turn failed                                                                                                       | `sessionId, message, code?, promptId?`; same `promptId` correlation mechanism.                                                                                                                   |
-| `session_rewound`     | S->C      | `POST /session/:id/rewind` succeeded                                                                                | `sessionId, promptId, targetTurnIndex, filesChanged[], filesFailed[], originatorClientId?`                                                                                                       |
-| `session_branched`    | S->C      | `POST /session/:id/branch` created a branch from an existing session                                                | `sourceSessionId, newSessionId, displayName, originatorClientId?`                                                                                                                                |
-| `followup_suggestion` | S->C      | ACP child generated ghost-text follow-up suggestions after `end_turn`, forwarded over per-session SSE               | `sessionId, suggestion, promptId`; wire only carries suggestions whose `getFilterReason()===null`. Clients render them as input-placeholder ghost text and invalidate them on next `sendPrompt`. |
-| `user_shell_command`  | S->C      | User started a shell command through `POST /session/:id/shell`; fanned out to other subscribers in the same session | `sessionId, command, shellId, originatorClientId?`. There is no typed `DaemonXxxData` interface yet; `asKnownDaemonEvent` returns `undefined` and the UI normalizer parses it ad hoc.            |
-| `user_shell_result`   | S->C      | Result of the shell command above                                                                                   | `sessionId, shellId, exitCode, output, aborted`. Same ad hoc parsing note as `user_shell_command`.                                                                                               |
+| Type                  | Direction | Trigger                                                                                                             | Key payload fields                                                                                                                                                                                    |
+| --------------------- | --------- | ------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `prompt_cancelled`    | S->C      | Prompt was cancelled through explicit `cancelSession` route **or** originator SSE disconnect                        | Envelope stamps `originatorClientId` for the canceling client. This means "cancellation requested", not "cancellation confirmed". Peer subscribers learn that the prompt has ended.                   |
+| `turn_complete`       | S->C      | A turn completed successfully                                                                                       | `sessionId, stopReason, promptId?, branchPoint?`. `promptId` links to non-blocking prompt responses (`202`). Eligible completed turns include `branchPoint: { assistantRecordUuid, checkpointUuid }`. |
+| `turn_error`          | S->C      | A turn failed                                                                                                       | `sessionId, message, code?, promptId?`; same `promptId` correlation mechanism.                                                                                                                        |
+| `session_rewound`     | S->C      | `POST /session/:id/rewind` succeeded                                                                                | `sessionId, promptId, targetTurnIndex, filesChanged[], filesFailed[], originatorClientId?`                                                                                                            |
+| `session_branched`    | S->C      | Legacy compatibility event; the current branch endpoint returns its result directly and does not publish this event | `sourceSessionId, newSessionId, displayName, originatorClientId?`. Readers retain support for older producers.                                                                                        |
+| `followup_suggestion` | S->C      | ACP child generated ghost-text follow-up suggestions after `end_turn`, forwarded over per-session SSE               | `sessionId, suggestion, promptId`; wire only carries suggestions whose `getFilterReason()===null`. Clients render them as input-placeholder ghost text and invalidate them on next `sendPrompt`.      |
+| `user_shell_command`  | S->C      | User started a shell command through `POST /session/:id/shell`; fanned out to other subscribers in the same session | `sessionId, command, shellId, originatorClientId?`. There is no typed `DaemonXxxData` interface yet; `asKnownDaemonEvent` returns `undefined` and the UI normalizer parses it ad hoc.                 |
+| `user_shell_result`   | S->C      | Result of the shell command above                                                                                   | `sessionId, shellId, exitCode, output, aborted`. Same ad hoc parsing note as `user_shell_command`.                                                                                                    |
 
 ## Architecture
 

integration-tests/cli/qwen-serve-streaming.test.ts

@@ -955,6 +955,78 @@ describePOSIX('qwen serve — Last-Event-ID resume', () => {
   }, 60_000);
 });
 
+describePOSIX('qwen serve — historical Assistant response branch', () => {
+  it('creates, opens, and continues a branch through the real daemon', async () => {
+    const source = await client.createOrAttachSession({
+      workspaceCwd: workspaceDir,
+      sessionScope: 'thread',
+    });
+    const first = await client.prompt(source.sessionId, {
+      prompt: [{ type: 'text', text: 'historical branch turn one' }],
+    });
+    expect(first.branchPoint).toBeDefined();
+    if (!first.branchPoint) return;
+
+    await client.prompt(source.sessionId, {
+      prompt: [{ type: 'text', text: 'historical branch turn two' }],
+    });
+    await client.prompt(source.sessionId, {
+      prompt: [{ type: 'text', text: 'historical branch turn three' }],
+    });
+
+    const branched = await client.branchSession(source.sessionId, {
+      atRecordId: first.branchPoint.checkpointUuid,
+    });
+    const branchBeforeContinue = await client.getSessionTranscriptPage(
+      branched.sessionId,
+      { limit: 500 },
+    );
+    const branchBeforeText = JSON.stringify(branchBeforeContinue.events);
+    expect(branchBeforeText).toContain('historical branch turn one');
+    expect(branchBeforeText).not.toContain('historical branch turn two');
+    expect(branchBeforeText).not.toContain('historical branch turn three');
+
+    const sourceAfterBranch = await client.getSessionTranscriptPage(
+      source.sessionId,
+      { limit: 500 },
+    );
+    const sourceText = JSON.stringify(sourceAfterBranch.events);
+    expect(sourceText).toContain('historical branch turn one');
+    expect(sourceText).toContain('historical branch turn two');
+    expect(sourceText).toContain('historical branch turn three');
+
+    const loadedBranch = await client.loadSession(branched.sessionId);
+    await client.prompt(
+      branched.sessionId,
+      {
+        prompt: [{ type: 'text', text: 'continue the historical branch' }],
+      },
+      undefined,
+      loadedBranch.clientId,
+    );
+    const branchAfterContinue = await client.getSessionTranscriptPage(
+      branched.sessionId,
+      { limit: 500 },
+    );
+    expect(JSON.stringify(branchAfterContinue.events)).toContain(
+      'continue the historical branch',
+    );
+
+    // The source session must stay untouched by the fork's continuation.
+    const sourceAfterContinue = await client.getSessionTranscriptPage(
+      source.sessionId,
+      { limit: 500 },
+    );
+    const sourceAfterContinueText = JSON.stringify(sourceAfterContinue.events);
+    expect(sourceAfterContinueText).not.toContain(
+      'continue the historical branch',
+    );
+    expect(sourceAfterContinueText).toContain('historical branch turn one');
+    expect(sourceAfterContinueText).toContain('historical branch turn two');
+    expect(sourceAfterContinueText).toContain('historical branch turn three');
+  }, 90_000);
+});
+
 describePOSIX('qwen serve — daemon Todo Stop Guard replay', () => {
   it('continues after prompt admission without an SSE client and replays the bounded attempts', async () => {
     const session = await client.createOrAttachSession({