perf(sessions): closing a session no longer waits on the server

Closing a tab took ~0.55-0.7s on an idle machine, more with child
processes or recently active subagents. Most of it was waiting:

- The web UI kept the tab until DELETE returned, then removed it on the
  100ms tab-render debounce, which every session update restarts.
  closeSession() is now optimistic: the tab, tile and split go and the
  next session is selected before the request is sent, rendered at
  once. A refused delete (checked with a GET, since a delete can land
  and lose its reply) puts the row back at its old index with the
  error toast. This also fixes a latent bug: _apiDelete never throws,
  so an HTTP error used to report "Session closed" while the session
  kept running. SSE upserts skip ids that are being closed.
- The kill path slept fixed intervals (100ms PTY grace, 200ms for the
  pane's children, 100ms for the process group) and verified in 100ms
  steps. waitForProcessesExit() (utils/process-exit-wait.ts) keeps
  every deadline but returns once the processes are gone, counting a
  zombie as exited. Signal decisions keep kill(pid, 0).
- tmux kill-session and the pane-pid lookup ran via execSync, freezing
  the server for ~70ms per close. Now async.
- killSubagentsForSession() ran a full `pgrep -f claude` scan per
  active/idle subagent (~85ms each with ~100 matching processes). It
  now scans once for all of them.

Measured on an isolated instance: click to tab gone 540-690ms -> 58-95ms;
DELETE of a claude session ~450ms -> ~200-260ms.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/session.ts

@@ -106,6 +106,7 @@ import {
   getClaudeBinaryPath,
   spawnPtyWithHelperRepair,
   resolveLocalShell,
+  waitForProcessesExit,
 } from './utils/index.js';
 import {
   MAX_TERMINAL_BUFFER_SIZE,
@@ -179,7 +180,10 @@ const WIRE_ACTIVITY_SETTLE_MS = 15_000;
 
 // Note: Auto-compact/clear timing constants moved to session-auto-ops.ts
 
-/** Graceful shutdown delay when stopping session (100ms) */
+/**
+ * Longest the PTY process gets to exit on SIGTERM before SIGKILL when stopping a
+ * session. A deadline, not a sleep: stop() moves on as soon as it has exited.
+ */
 const GRACEFUL_SHUTDOWN_DELAY_MS = 100;
 
 // Conversations kept in a pane's chain. A pane that /clears repeatedly would
@@ -4801,8 +4805,10 @@ export class Session extends EventEmitter {
           console.warn('[Session] Failed to send SIGTERM to PTY process (may already be dead):', err);
         }
 
-        // Give it a moment to terminate gracefully
-        await new Promise((resolve) => setTimeout(resolve, GRACEFUL_SHUTDOWN_DELAY_MS));
+        // Give it a moment to terminate gracefully. For a tmux-backed session this
+        // is the attach client, gone within a few ms of SIGTERM, and this used to be
+        // a fixed 100ms sleep on every close.
+        if (pid) await waitForProcessesExit([pid], { timeoutMs: GRACEFUL_SHUTDOWN_DELAY_MS });
 
         // Force kill with SIGKILL if still alive
         try {