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.

test/session.test.ts

@@ -48,6 +48,7 @@ const CFG: Config = {
   scrollbackBytes: 2 * 1024 * 1024,
   maxPayloadBytes: 1024 * 1024,
   wsPath: '/term',
+  useTmux: false,
   allowedOrigins: [],
 };