fix(web): address self-review findings on #103 (master-safe defaults + hardening)

Make the branch genuinely master-mergeable and fix several review findings:

- Defaults are now prod-safe: CODEMAN_INSTANCE defaults to '' (→ ~/.codeman,
  -L codeman) and the web port back to 3000, so an existing install upgrades
  cleanly. Port also honors a new CODEMAN_PORT env var. Run the beta isolated
  alongside prod with scripts/run-beta.sh (CODEMAN_INSTANCE=beta + PORT 5000).
- .gitignore: anchor the root `public` symlink rule to `/public` (a bare
  `public` also swallowed src/web/public, silently un-staging new web assets);
  ignore the gesture wasm/model binaries explicitly instead.
- span-displays: add a macOS-only guard (400 elsewhere instead of spawning a
  bash that fails invisibly); extract resolveSpanUrl() for unit testing.
- server.ts: memoize asset-version stat() calls (~1s TTL) so each index render
  doesn't re-stat every script/link tag.
- styles.css: hide the multi-monitor button in solo (detached) windows.
- app.js: require two consecutive unanswered roll-calls before redocking, so a
  timer-throttled background popup isn't wrongly un-marked.
- index.html: make the "skip to terminal" link base-href-safe (onclick scroll)
  so it doesn't navigate to the dashboard from a /session/:id window.
- Tests: test/config/instance.test.ts, test/routes/system-span-displays.test.ts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -30,7 +30,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 2. **Frontend changes**: Use Playwright to load the page and assert the UI renders correctly. Use `waitUntil: 'domcontentloaded'` (not `networkidle` — SSE keeps the connection open). Wait 3-4s for polling/async data to populate, then check element visibility, text content, and CSS values
 3. **Only after verification passes**, proceed with COM
 
-The production server caches static files for 1 year, `immutable` (`maxAge: '1y'` in `server.ts`). To avoid stale frontend after a deploy, `renderIndexHtml` runs `cacheBustAssets(html)` — it appends `?v=<mtime>` to **every same-origin `.js`/`.css`** reference (re-stat'd per render; external/already-versioned/missing refs untouched). Because `index.html` is served `no-cache`, a **normal reload now picks up edited modules/styles — no hard refresh needed** (the gesture bundle is injected separately with its own `?v=`). If you add an asset referenced by an *absolute* URL or from JS rather than a `<script>/<link>` tag, it won't be auto-busted.
+The production server caches static files for 1 year, `immutable` (`maxAge: '1y'` in `server.ts`). To avoid stale frontend after a deploy, `renderIndexHtml` runs `cacheBustAssets(html)` — it appends `?v=<mtime>` to **every same-origin `.js`/`.css`** reference (mtime memoized ~1s so a burst of renders is cheap; external/already-versioned/missing refs untouched). Because `index.html` is served `no-cache`, a **normal reload now picks up edited modules/styles — no hard refresh needed** (the gesture bundle is injected separately with its own `?v=`). If you add an asset referenced by an *absolute* URL or from JS rather than a `<script>/<link>` tag, it won't be auto-busted.
 
 ## COM Shorthand (Deployment)
 
@@ -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: `5000` on this `beta/session-detach` branch (`3000` on master). Commands not in Quick Reference:
+`npm run dev` = dev server. Default port: `3000` (override with `--port` or the `CODEMAN_PORT` env var). To run this beta isolated alongside a prod Codeman, use `scripts/run-beta.sh` (sets `CODEMAN_INSTANCE=beta` + `CODEMAN_PORT=5000`). Commands not in Quick Reference:
 
 | Task | Command |
 |------|---------|
@@ -98,7 +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 — `$HOME` isolation is NOT enough (tmux is system-global). To run two instances, give each a distinct `CODEMAN_INSTANCE` (scopes BOTH dir+socket: `~/.codeman-<name>` + `-L codeman-<name>`), or set `CODEMAN_TMUX_SOCKET` + `CODEMAN_DATA_DIR` individually. **This `beta/session-detach` branch defaults to `CODEMAN_INSTANCE=beta` and port 5000** so it coexists with a prod Codeman out of the box. Any new `~/.codeman/...` path MUST go through `dataPath()`, never `join(homedir(), '.codeman', …)`.
+- **Instance isolation / multi-instance attach danger** — data dir (`~/.codeman`) and tmux socket (`tmux -L codeman`) are PROCESS-WIDE and shared by every Codeman on the machine, derived from `CODEMAN_INSTANCE` via `src/config/instance.ts` (`getDataDir()`/`dataPath()`/`DEFAULT_TMUX_SOCKET`). ⚠️ A 2nd instance on the SAME socket **discovers and attaches PTYs to the first instance's live sessions** (`tmux -L codeman attach-session …`), resizing/mutating them — `$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. **`CODEMAN_INSTANCE` defaults to empty = the production layout (`~/.codeman`, `-L codeman`, port 3000)**, so this branch is safe to ship to master without disturbing existing installs. To run THIS beta alongside prod, launch with `scripts/run-beta.sh` (`CODEMAN_INSTANCE=beta` + `CODEMAN_PORT=5000`) — it never collides with prod's data dir/socket/port. 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.