fix: prevent zombie processes with proper process lifecycle management (#1306)

* fix: prevent zombie processes with proper process lifecycle management

- Await proc.exited for fire-and-forget spawns in tmux-utils.ts
- Remove competing process.exit() calls from LSP client and skill-mcp-manager
  signal handlers to let background-agent manager coordinate final exit
- Await process exit after kill() in interactive-bash timeout handler
- Await process exit after kill() in LSP client stop() method

These changes ensure spawned processes are properly reaped and prevent
orphan/zombie processes when running with tmux integration.

* fix: address Copilot review comments on process cleanup

- LSP cleanup: use async/sync split with Promise.allSettled for proper subprocess cleanup
- LSP stop(): make idempotent by nulling proc before await to prevent race conditions
- Interactive-bash timeout: use .then()/.catch() pattern instead of async callback to avoid unhandled rejections
- Skill-mcp-manager: use void+catch pattern for fire-and-forget signal handlers

* fix: address remaining Copilot review comments

- interactive-bash: reject timeout immediately, fire-and-forget zombie cleanup
- skill-mcp-manager: update comments to accurately describe signal handling strategy

* fix: address additional Copilot review comments

- LSP stop(): add 5s timeout to prevent indefinite hang on stuck processes
- tmux-utils: log warnings when pane title setting fails (both spawn/replace)
- BackgroundManager: delay process.exit() to next tick via setImmediate to allow other signal handlers to complete cleanup

* fix: address code review findings

- Increase exit delay from setImmediate to 100ms setTimeout to allow async cleanup
- Use asyncCleanup for SIGBREAK on Windows for consistency with SIGINT/SIGTERM
- Add try/catch around stderr read in spawnTmuxPane for consistency with replaceTmuxPane

* fix: address latest Copilot review comments

- LSP stop(): properly clear timeout when proc.exited wins the race
- BackgroundManager: use process.exitCode before delayed exit for cleaner shutdown
- spawnTmuxPane: remove redundant log import, reuse existing one

* fix: address latest Copilot review comments

- LSP stop(): escalate to SIGKILL on timeout, add logging
- tmux spawnTmuxPane/replaceTmuxPane: drain stderr immediately to avoid backpressure

* fix: address latest Copilot review comments

- Add .catch() to asyncCleanup() signal handlers to prevent unhandled rejections
- Await proc.exited after SIGKILL with 1s timeout to confirm termination

* fix: increase exit delay to 6s to accommodate LSP cleanup

LSP cleanup can take up to 5s (timeout) + 1s (SIGKILL wait), so the exit
delay must be at least 6s to ensure child processes are properly reaped.

src/features/background-agent/manager.ts

@@ -1396,7 +1396,10 @@ function registerProcessSignal(
   const listener = () => {
     handler()
     if (exitAfter) {
-      process.exit(0)
+      // Set exitCode and schedule exit after delay to allow other handlers to complete async cleanup
+      // Use 6s delay to accommodate LSP cleanup (5s timeout + 1s SIGKILL wait)
+      process.exitCode = 0
+      setTimeout(() => process.exit(), 6000)
     }
   }
   process.on(signal, listener)

src/features/skill-mcp-manager/manager.ts

@@ -114,23 +114,15 @@ export class SkillMcpManager {
       this.pendingConnections.clear()
     }
 
-    // Note: 'exit' event is synchronous-only in Node.js, so we use 'beforeExit' for async cleanup
-    // However, 'beforeExit' is not emitted on explicit process.exit() calls
-    // Signal handlers are made async to properly await cleanup
+    // Note: Node's 'exit' event is synchronous-only, so we rely on signal handlers for async cleanup.
+    // Signal handlers invoke the async cleanup function and ignore errors so they don't block or throw.
+    // Don't call process.exit() here - let the background-agent manager handle the final process exit.
+    // Use void + catch to trigger async cleanup without awaiting it in the signal handler.
 
-    process.on("SIGINT", async () => {
-      await cleanup()
-      process.exit(0)
-    })
-    process.on("SIGTERM", async () => {
-      await cleanup()
-      process.exit(0)
-    })
+    process.on("SIGINT", () => void cleanup().catch(() => {}))
+    process.on("SIGTERM", () => void cleanup().catch(() => {}))
     if (process.platform === "win32") {
-      process.on("SIGBREAK", async () => {
-        await cleanup()
-        process.exit(0)
-      })
+      process.on("SIGBREAK", () => void cleanup().catch(() => {}))
     }
   }