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/state-store.ts

@@ -39,6 +39,7 @@ import {
   TokenUsageEntry,
 } from './types.js';
 import { Debouncer, MAX_SESSION_TOKENS } from './utils/index.js';
+import { dataPath } from './config/instance.js';
 
 /** Debounce delay for batching state writes (ms) */
 const SAVE_DEBOUNCE_MS = 500;
@@ -105,7 +106,7 @@ export class StateStore {
       }
     }
 
-    this.filePath = filePath || join(homedir(), '.codeman', 'state.json');
+    this.filePath = filePath || dataPath('state.json');
     this.ralphStatePath = this.filePath.replace('.json', '-inner.json');
     this.state = this.load();
     this.state.config.stateFilePath = this.filePath;