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/config/instance.ts

@@ -0,0 +1,60 @@
+/**
+ * @fileoverview Per-instance isolation: data directory + tmux socket.
+ *
+ * Codeman keeps all runtime state under `~/.codeman` and runs its tmux sessions
+ * on a dedicated socket (`tmux -L codeman`). Both are PROCESS-WIDE and SHARED by
+ * every Codeman instance on the machine — so a second instance pointed at the
+ * same socket will discover and attach to the first instance's live sessions,
+ * and two instances sharing `~/.codeman/state.json` will clobber each other.
+ *
+ * To let a beta build coexist with a production one, this module derives both
+ * the data dir and the tmux socket from a single "instance" name:
+ *   - default (this branch): `beta` → `~/.codeman-beta` + `tmux -L codeman-beta`
+ *   - `CODEMAN_INSTANCE=`     (empty) → `~/.codeman`     + `tmux -L codeman`   (prod layout)
+ *   - `CODEMAN_INSTANCE=foo`          → `~/.codeman-foo`  + `tmux -L codeman-foo`
+ *
+ * Individual overrides still win: `CODEMAN_DATA_DIR` (absolute data dir) and
+ * `CODEMAN_TMUX_SOCKET` (socket name, validated in tmux-manager).
+ */
+
+import { homedir } from 'node:os';
+import { join } from 'node:path';
+import { mkdirSync } from 'node:fs';
+
+/**
+ * Instance name. Empty string = production layout (`~/.codeman`, `-L codeman`).
+ * Defaults to `beta` on the beta/session-detach branch so it never collides
+ * with a production Codeman. Set `CODEMAN_INSTANCE=` (empty) to opt back into
+ * the production layout.
+ */
+export const CODEMAN_INSTANCE = process.env.CODEMAN_INSTANCE ?? 'beta';
+
+const INSTANCE_SUFFIX = CODEMAN_INSTANCE ? `-${CODEMAN_INSTANCE}` : '';
+
+/** Default tmux socket for this instance. `CODEMAN_TMUX_SOCKET` still overrides. */
+export const DEFAULT_TMUX_SOCKET = `codeman${INSTANCE_SUFFIX}`;
+
+let _ensured = false;
+
+/**
+ * Absolute path to this instance's data directory (created on first use). All
+ * persisted state (`state.json`, `mux-sessions.json`, settings, push keys,
+ * lifecycle log, screenshots, certs, …) lives here.
+ */
+export function getDataDir(): string {
+  const dir = process.env.CODEMAN_DATA_DIR || join(homedir(), `.codeman${INSTANCE_SUFFIX}`);
+  if (!_ensured) {
+    try {
+      mkdirSync(dir, { recursive: true });
+      _ensured = true;
+    } catch {
+      /* best-effort; individual writers also mkdir as needed */
+    }
+  }
+  return dir;
+}
+
+/** Join one or more segments onto this instance's data directory. */
+export function dataPath(...segments: string[]): string {
+  return join(getDataDir(), ...segments);
+}