feat(channels): add BTW side questions to Channel conversations

docs/users/features/channels/overview.md

@@ -489,6 +489,7 @@ Channels support slash commands. These are handled locally (no agent round-trip)
 - `/help` — List available commands
 - `/clear` — Clear your session and start fresh (aliases: `/reset`, `/new`)
 - `/status` — Show session info and access policy
+- `/btw <question>` — Ask a side question without interrupting the current task; text-only questions up to 4096 characters, requires an agent connection with side-question support
 - `/sessions [all]` — List open named tasks, or include closed tasks; available only with `multiSession: true`
 - `/session current` — Show the selected named task
 - `/session new <name>` — Create and select a shared-workspace task

packages/channels/base/src/ChannelAgentBridge.ts

@@ -207,6 +207,16 @@ export interface ChannelAgentBridge {
     command: string,
     signal?: AbortSignal,
   ): Promise<{ exitCode: number | null; output: string; aborted: boolean }>;
+  /**
+   * Answer a side question without interrupting the session's active turn.
+   * The result must echo the request's sessionId. Bridges whose agent
+   * connection cannot answer side questions omit it and channels fail closed.
+   */
+  btw?(
+    sessionId: string,
+    question: string,
+    signal?: AbortSignal,
+  ): Promise<{ sessionId: string; answer: string | null }>;
   listSessions?(): BridgeSessionInfo[];
   registerChannelLoopToolHandler?(handler: ChannelLoopToolHandler): void;
 }