fix(desktop): add dismiss button for finished/timed-out subagent rows

[wen0531]

Problem

Subagent status rows in the composer status stack have no dismiss (X) button after completing, failing, or timing out. Only background type rows get the close affordance — subagent items always return null from the action selector, leaving finished cards permanently stuck in the UI until the app is restarted.

Root Cause

In status-row.tsx, the action variable only checks for item.type === background:

const action =
    item.type === background
      ? running
        ? onStop && { ... }
        : onDismiss && { ... }
      : null  // ← subagent always gets null

The onDismiss callback is passed from the parent for all item types, but subagent items never use it.

Fix (3 files, +23/-2 lines)

1. status-row.tsx — extend action condition

- item.type === background
+ item.type === background || item.type === subagent

2. subagents.ts — add dismissSubagent()

New function that removes a single subagent by ID from $subagentsBySession, paralleling the existing dismissBackgroundProcess for background items.

3. index.tsx — dispatch to correct store

onDismiss={sessionId
  ? id => (item.type === subagent
      ? dismissSubagent(sessionId, id)
      : dismissBackgroundProcess(sessionId, id))
  : undefined}

Behavior

Subagent state	Button shown	Action
running	✕ Stop	Kills the process, then removes the row
completed / failed / interrupted / timeout	✕ Dismiss	Removes the row from display

Verification

• TypeScript compiles cleanly (tsc --noEmit, 0 errors)
• Desktop builds successfully (npm run build)
• Tested locally: finished subagent rows now show an X button that removes them

[spfcraze]

This was generated by AI during triage.

Summary:
Every subagent row renders with a hardcoded state: 'running' (subToItem, composer-status.ts:136), so the new Dismiss branch is unreachable and the Stop branch sends the subagent id to process.kill, which resolves against the background-process registry and returns 4044 — the row stays and an error surfaces.

Problems:

• subToItem (composer-status.ts:136) sets state: 'running' on every subagent row, and the stack computed only pushes running/queued subagents (composer-status.ts:212), so in status-row.tsx the running ? onStop : onDismiss ternary always selects the Stop branch for subagent rows; the new dismissSubagent path wired in index.tsx never renders.
• The Stop button that does render calls stopBackgroundProcess → process.kill with the subagent's id (composer-status.ts:412). process.kill resolves process_registry.get(proc_id) (tui_gateway/methods_tools.py:74) and returns 4044 "no such process" when the lookup misses (methods_tools.py:78); stopBackgroundProcess then notifies "Could not stop the process" and only dismisses on a confirmed kill (composer-status.ts:410-418). Net effect: error toast, row stays.
• The premise that finished subagent rows are stuck until restart does not match main: the stack computed filters subagents to running/queued (composer-status.ts:212) and pruneFinishedSessionSubagents runs at message.start (gateway-event.ts:543). The genuinely stuck case is the timeout/error → running normalization in asStatus (subagents.ts:59), tracked in Desktop leaves timed-out subagents stuck as running #73728, with PR fix(desktop): normalise timeout/error subagent statuses to terminal (#73728) #73859 addressing it — this PR does not reference either.

Solution:
Carry the real subagent status into the row state so the Dismiss branch is reachable, and do not send subagent ids to process.kill — subagent rows are keyed by subagent id from subagent.* events, not by process.list entries; the desktop's existing stop path for subagent-bearing turns is session-level (clearSessionSubagents + session.interrupt, use-prompt-actions/index.ts:619-630). The stuck-row symptom itself is the status normalization in #73728/#73859.

---

Checked against aa9e551 — the tip of fix/subagent-dismiss-button when this was written — and 55505be, main at the same moment.