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>

CLAUDE.md

@@ -72,7 +72,7 @@ Codeman is a Claude Code session manager with web interface and autonomous Ralph
 
 ## Additional Commands
 
-`npm run dev` = dev server. Default port: `3000`. Commands not in Quick Reference:
+`npm run dev` = dev server. Default port: `5000` on this `beta/session-detach` branch (`3000` on master). Commands not in Quick Reference:
 
 | Task | Command |
 |------|---------|
@@ -98,6 +98,7 @@ Codeman is a Claude Code session manager with web interface and autonomous Ralph
 - **Dual-CLI prefix discipline** — Codeman supports both Claude Code and OpenCode (`claude-cli-resolver.ts` / `opencode-cli-resolver.ts`); env-var prefix is CLI-specific (`CLAUDE_CODE_*` vs `OPENCODE_*`) and the allowlist in `schemas.ts` enforces this. When adding settings, decide which CLI(s) it applies to and gate the env export accordingly — don't blindly forward both prefixes. See `docs/opencode-integration.md` for the OpenCode resolver design
 - **Zod `.optional()` rejects `null`** — accepts `undefined` only. When the frontend builds a request body with `JSON.stringify`, an explicit `null` field is preserved on the wire and fails validation with `INVALID_INPUT`. Convert `null` → `undefined` before stringifying (e.g. `field: value ?? undefined`), or declare the schema `.nullish()`. Real bugs caused: 0.6.4 (`durationMinutes` for ∞ respawn), and the same shape pattern hit `opusContext1mEnabled` in 0.6.3
 - **`xterm-zerolag-input` is duplicated** — the local-echo overlay lives in BOTH `packages/xterm-zerolag-input/src/` (published to npm as a standalone library for external consumers — see README "Published Packages") AND inline inside `src/web/public/app.js` (runtime copy the web UI actually loads, since the page ships as plain JS without a bundler). Any change to overlay behavior MUST be applied to both, or dev and prod diverge — and a public API break in the package warrants a separate version bump for `xterm-zerolag-input` in the changeset. Always test on mobile after touching it. See `docs/local-echo-overlay-plan.md`.
+- **Instance isolation / multi-instance attach danger** — data dir (`~/.codeman`) and tmux socket (`tmux -L codeman`) are PROCESS-WIDE and shared by every Codeman on the machine, derived from `CODEMAN_INSTANCE` via `src/config/instance.ts` (`getDataDir()`/`dataPath()`/`DEFAULT_TMUX_SOCKET`). ⚠️ A 2nd instance on the SAME socket **discovers and attaches PTYs to the first instance's live sessions** (`tmux -L codeman attach-session …`), resizing/mutating them — `$HOME` isolation is NOT enough (tmux is system-global). To run two instances, give each a distinct `CODEMAN_INSTANCE` (scopes BOTH dir+socket: `~/.codeman-<name>` + `-L codeman-<name>`), or set `CODEMAN_TMUX_SOCKET` + `CODEMAN_DATA_DIR` individually. **This `beta/session-detach` branch defaults to `CODEMAN_INSTANCE=beta` and port 5000** so it coexists with a prod Codeman out of the box. Any new `~/.codeman/...` path MUST go through `dataPath()`, never `join(homedir(), '.codeman', …)`.
 
 **Import conventions**: Utils from `./utils`, types from `./types` (barrel), config from specific `./config/*` files.