feat(v0.3): H1 — tmux keepalive (sessions survive a server restart)

- src/session/tmux.ts: sync tmux CLI wrappers (available/has/kill), best-effort
- config: useTmux from USE_TMUX env (1/0/auto→detect tmux on PATH)
- session: when useTmux, spawn 'tmux new-session -A -s web_<id> <shell>' (the
  node-pty proc is a tmux CLIENT); createSession takes an optional id for
  re-attach; Session.tmuxName; kill() runs tmux kill-session
- manager: handleAttach re-attaches to a surviving 'web_<id>' tmux session after
  a restart (Case 3.5); shutdown kills only the client pty for tmux sessions so
  the shell keeps running
- tests: USE_TMUX:0 in shared integration cfg (no tmux leakage); unit CFGs
  useTmux:false; integration 'H1' (real tmux): set var → restart server →
  re-attach → var survived. 208 tests green.

src/config.ts

@@ -8,6 +8,15 @@
 
 import os from 'node:os'
 import type { Config, EnvLike } from './types.js'
+import { tmuxAvailable } from './session/tmux.js'
+
+/** USE_TMUX: '1'/'true'/'on' → on, '0'/'false'/'off' → off, else auto-detect tmux. */
+function resolveUseTmux(raw: string | undefined): boolean {
+  const v = raw?.trim().toLowerCase()
+  if (v === '1' || v === 'true' || v === 'on') return true
+  if (v === '0' || v === 'false' || v === 'off') return false
+  return tmuxAvailable() // unset / 'auto'
+}
 
 // ── constants ─────────────────────────────────────────────────────────────────
 
@@ -141,6 +150,8 @@ export function loadConfig(env: EnvLike): Config {
 
   const wsPath = env['WS_PATH'] ?? DEFAULT_WS_PATH
 
+  const useTmux = resolveUseTmux(env['USE_TMUX'])
+
   const allowedOrigins = deriveAllowedOrigins(port, env['ALLOWED_ORIGINS'])
 
   return Object.freeze({
@@ -152,6 +163,7 @@ export function loadConfig(env: EnvLike): Config {
     scrollbackBytes,
     maxPayloadBytes,
     wsPath,
+    useTmux,
     allowedOrigins,
   } satisfies Config)
 }