fix(tmux): send Ctrl+C before kill-pane and respawn-pane to prevent orphaned processes (#1329)

* fix(tmux): send Ctrl+C before kill-pane and respawn-pane to prevent orphaned processes

* fix(tmux-subagent): prevent premature pane closure with stability detection

Implements stability detection pattern from background-agent to prevent
tmux panes from closing while agents are still working (issue #1330).

Problem: Session status 'idle' doesn't mean 'finished' - agent may still
be thinking/reasoning. Previous code closed panes immediately on idle.

Solution:
- Require MIN_STABILITY_TIME_MS (10s) before stability detection activates
- Track message count changes to detect ongoing activity
- Require STABLE_POLLS_REQUIRED (3) consecutive polls with same message count
- Double-check session status before closing

Changes:
- types.ts: Add lastMessageCount and stableIdlePolls to TrackedSession
- manager.ts: Implement stability detection in pollSessions()
- manager.test.ts: Add 4 tests for stability detection behavior

* test(tmux-subagent): improve stability detection tests to properly verify age gate

- First test now sets session age >10s and verifies 3 polls don't close
- Last test now does 5 polls to prove age gate prevents closure
- Added comments explaining what each poll does

src/shared/tmux/tmux-utils.ts

@@ -173,6 +173,17 @@ export async function closeTmuxPane(paneId: string): Promise<boolean> {
     return false
   }
 
+  // Send Ctrl+C to trigger graceful exit of opencode attach process
+  log("[closeTmuxPane] sending Ctrl+C for graceful shutdown", { paneId })
+  const ctrlCProc = spawn([tmux, "send-keys", "-t", paneId, "C-c"], {
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+  await ctrlCProc.exited
+
+  // Brief delay for graceful shutdown
+  await new Promise((r) => setTimeout(r, 250))
+
   log("[closeTmuxPane] killing pane", { paneId })
   
   const proc = spawn([tmux, "kill-pane", "-t", paneId], {
@@ -214,6 +225,18 @@ export async function replaceTmuxPane(
     return { success: false }
   }
 
+  // Send Ctrl+C to trigger graceful exit of existing opencode attach process
+  // Note: No delay here - respawn-pane -k will handle any remaining process.
+  // We send Ctrl+C first to give the process a chance to exit gracefully,
+  // then immediately respawn. This prevents orphaned processes while avoiding
+  // the race condition where the pane closes before respawn-pane runs.
+  log("[replaceTmuxPane] sending Ctrl+C for graceful shutdown", { paneId })
+  const ctrlCProc = spawn([tmux, "send-keys", "-t", paneId, "C-c"], {
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+  await ctrlCProc.exited
+
   const opencodeCmd = `opencode attach ${serverUrl} --session ${sessionId}`
 
   const proc = spawn([tmux, "respawn-pane", "-k", "-t", paneId, opencodeCmd], {