feat(web): session detach/undock + beta instance isolation (port 5000)

Detach a session tab into its own browser window and back.

Detach/undock:
- GET /session/:id serves the SPA in "solo mode", reusing the existing
  client (terminal, local-echo overlay, reconnect) so no terminal code is
  duplicated. One PTY already fans out to N SSE/WS clients, so a detached
  window is just another live client — no server fan-out work was needed.
- A pop-out icon per tab; detached tabs show a badge and focus the popup on
  click; closing the popup re-docks. Cross-window state via BroadcastChannel
  plus a WindowProxy poll, and survives a dashboard reload (roll-call).
  app.detachSession(id) is a single idempotent entry point (future gesture
  hook). <base href="/"> so relative assets resolve under /session/:id.

Beta-branch isolation (so it can run alongside a prod Codeman):
- Default port 3000 -> 5000.
- New src/config/instance.ts derives the data dir and tmux socket from
  CODEMAN_INSTANCE (default "beta"): ~/.codeman-beta + tmux -L codeman-beta.
  Every ~/.codeman path now goes through dataPath()/getDataDir() (state,
  mux-sessions, settings, push keys, lifecycle log, screenshots, certs,
  linked-cases, subagent window state). Overridable via CODEMAN_INSTANCE /
  CODEMAN_DATA_DIR / CODEMAN_TMUX_SOCKET. Prevents a second instance from
  discovering and attaching PTYs to the first instance's live tmux sessions.

Verified: tsc / eslint / prettier / lockfile clean; Playwright E2E (27 checks)
for detach/solo/redock; default isolation confirmed to see zero real sessions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/cli.ts

@@ -483,7 +483,7 @@ program
 program
   .command('web')
   .description('Start the web interface')
-  .option('-p, --port <port>', 'Port to listen on', '3000')
+  .option('-p, --port <port>', 'Port to listen on', '5000')
   .option('--https', 'Enable HTTPS with self-signed certificate (only needed for remote access, not localhost)')
   .option('--title-hostname <hostname>', 'Override the hostname shown in the browser title')
   .action(async (options) => {