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/web/public/index.html

@@ -2,6 +2,10 @@
 <html lang="en">
 <head>
   <meta charset="UTF-8">
+  <!-- Resolve all relative assets against the site root so the same shell can be
+       served at /session/:id (detached single-session window) without 404ing
+       on relative <script>/<link> URLs. Must precede the first resource tag. -->
+  <base href="/">
   <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, viewport-fit=cover">
   <meta name="description" content="Claude Code session manager with web interface">
   <meta name="theme-color" content="#0a0a0a">
@@ -67,7 +71,11 @@
       <div class="session-tabs" id="sessionTabs" role="tablist" aria-label="Session tabs">
       </div>
 
+      <!-- Detached single-session window title (shown only in solo mode) -->
+      <div class="solo-session-title" id="soloSessionTitle" style="display: none;" aria-live="polite"></div>
+
       <div class="header-right">
+        <button class="btn-icon-header btn-solo-redock" id="soloRedockBtn" style="display: none;" onclick="window.close()" title="Re-dock to dashboard (close window)" aria-label="Re-dock session to dashboard">&#x229E;</button>
         <button class="tunnel-indicator" id="tunnelIndicator" style="display: none;" onclick="app.toggleTunnelPanel()" title="Cloudflare Tunnel" aria-label="Tunnel status">
           <span class="tunnel-dot"></span>
         </button>