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/styles.css

@@ -905,6 +905,96 @@ body {
   transform: rotate(45deg);
 }
 
+/* ===== Session detach / undock (beta/session-detach) ===================== */
+
+/* Pop-out (detach) icon on each tab — mirrors .tab-gear reveal-on-hover. */
+.session-tab .tab-detach {
+  opacity: 0;
+  width: 0;
+  padding: 0;
+  font-size: 0.95rem;
+  line-height: 1;
+  color: var(--text-muted);
+  cursor: pointer;
+  overflow: hidden;
+  transition: opacity 0.15s, width 0.15s, padding 0.15s;
+}
+.session-tab:hover .tab-detach {
+  opacity: 1;
+  width: auto;
+  padding: 0 0.3rem;
+}
+.session-tab .tab-detach:hover {
+  color: var(--accent-hover);
+  background: rgba(255, 255, 255, 0.1);
+  border-radius: 3px;
+}
+
+/* A tab whose session is popped out into its own window. */
+.session-tab.detached {
+  opacity: 0.72;
+}
+.session-tab.detached .tab-detach {
+  /* Keep the pop-out icon visible while detached as a re-focus affordance. */
+  opacity: 1;
+  width: auto;
+  padding: 0 0.3rem;
+  color: var(--accent-hover);
+}
+.session-tab .tab-detached-badge {
+  display: none;
+  align-items: center;
+  font-size: 0.55rem;
+  font-weight: 700;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+  padding: 1px 4px;
+  margin-left: 4px;
+  border-radius: 3px;
+  background: rgba(96, 165, 250, 0.18);
+  color: var(--accent-hover);
+}
+.session-tab.detached .tab-detached-badge {
+  display: inline-flex;
+}
+
+/* ===== Solo (detached single-session) window chrome ===================== */
+body.solo-mode .session-tabs,
+body.solo-mode .header-system-stats,
+body.solo-mode .header-tokens,
+body.solo-mode .btn-notifications,
+body.solo-mode .btn-lifecycle-log {
+  display: none !important;
+}
+.solo-session-title {
+  flex: 1;
+  min-width: 0;
+  font-weight: 600;
+  font-size: 13px;
+  color: var(--text);
+  padding: 0 12px;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+/* "Session unavailable" overlay for a solo window whose session has ended. */
+.solo-gone-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 5000;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 14px;
+  background: rgba(9, 9, 11, 0.92);
+  color: var(--text);
+  text-align: center;
+  padding: 24px;
+}
+.solo-gone-overlay h2 { margin: 0; font-size: 18px; }
+.solo-gone-overlay p { margin: 0; color: var(--text-muted); font-size: 13px; }
+
 /* Mode indicator on session tab */
 .session-tab .tab-mode {
   font-size: 0.6rem;