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/session-lifecycle-log.ts

@@ -10,9 +10,9 @@
 
 import { appendFile, readFile, writeFile } from 'node:fs/promises';
 import { existsSync, mkdirSync } from 'node:fs';
-import { dirname, join } from 'node:path';
-import { homedir } from 'node:os';
+import { dirname } from 'node:path';
 import type { LifecycleEventType, LifecycleEntry } from './types.js';
+import { dataPath } from './config/instance.js';
 
 const MAX_LINES = 10_000;
 const TRIM_TO = 8_000;
@@ -22,7 +22,7 @@ export class SessionLifecycleLog {
   private writeQueue: Promise<void> = Promise.resolve();
 
   constructor(filePath?: string) {
-    this.filePath = filePath || join(homedir(), '.codeman', 'session-lifecycle.jsonl');
+    this.filePath = filePath || dataPath('session-lifecycle.jsonl');
     const dir = dirname(this.filePath);
     if (!existsSync(dir)) {
       mkdirSync(dir, { recursive: true, mode: 0o700 });