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>

src/cli.ts

@@ -483,7 +483,7 @@ program
 program
   .command('web')
   .description('Start the web interface')
-  .option('-p, --port <port>', 'Port to listen on', '5000')
+  .option('-p, --port <port>', 'Port to listen on (env: CODEMAN_PORT)', process.env.CODEMAN_PORT || '3000')
   .option('--https', 'Enable HTTPS with self-signed certificate (only needed for remote access, not localhost)')
   .option('--title-hostname <hostname>', 'Override the hostname shown in the browser title')
   .action(async (options) => {