fix(usage): address code-review findings in plan-usage telemetry

Review of the plan-usage chip feature (commits since 1.0.0) surfaced several
issues; this fixes all confirmed findings:

- HIGH: applyStatusLineConfig clobbered a user's hand-authored statusLine on
  the enable path (the isOurs guard only protected disable). Now bails out when
  an existing statusLine isn't ours, on both the enable and disable paths.
- MED: StatusTelemetrySchema used z.optional() (rejects null) on Claude's
  undocumented statusline fields — a single stray null 400'd the entire POST and
  silently killed the chip's data feed. Switched the modeled fields to .nullish().
- MED: dropping the Token Count / Show Cost header toggles left their features
  reading settings.showTokenCount/showCost, but saveAppSettings rebuilds settings
  fresh from the DOM, dropping those keys and resetting them to defaults on every
  save (re-enabling the token chip with no UI to turn it off). Preserve the prior
  stored preference.
- telemetrySignature keyed on contextUsedPercentage (never displayed) and the raw
  unrounded %, churning a redundant SSE broadcast + localStorage write + identical
  chip re-render on every assistant message. Now keys on the rounded displayed
  window values only.
- Plan-usage chip flashed hidden on load (no server-side reveal): renderIndexHtml
  now strips header-plan-usage--hidden when enabled, matching btn-multimonitor;
  fixes the FOUC and makes the "server renders initial state" comments accurate.
- Serialize all settings.local.json read-modify-write writers in hooks-config via
  a shared per-path mutex (previously lock-free; concurrent session-create +
  settings-toggle on the same repo could lose writes).
- Hardened the chip's innerHTML against any future string field; removed the dead
  _latestPlanUsage field; clamped ctx% in the footer formatter; corrected the
  session-create comment (the path is add-only by design — a per-repo settings
  file is shared by sibling sessions).
- Tests: new test/routes/status-telemetry-routes.test.ts (route behavior, dedup,
  null-tolerance) + NaN/Infinity/fractional and signature-churn unit tests; made
  server-index-title.test.ts deterministic against the ambient settings.json.

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

src/web/public/settings-ui.js

@@ -1351,7 +1351,8 @@ Object.assign(CodemanApp.prototype, {
   async saveAppSettings() {
     // Gesture overlay is injected at page render (server-side), so a change to it
     // only takes effect on reload — remember the prior value to decide below.
-    const _prevGestureEnabled = (this.loadAppSettingsFromStorage().gestureControlEnabled ?? false) === true;
+    const _prev = this.loadAppSettingsFromStorage();
+    const _prevGestureEnabled = (_prev.gestureControlEnabled ?? false) === true;
     const settings = {
       defaultClaudeMdPath: document.getElementById('appSettingsClaudeMdPath').value.trim(),
       defaultWorkingDir: document.getElementById('appSettingsDefaultDir').value.trim(),
@@ -1394,6 +1395,16 @@ Object.assign(CodemanApp.prototype, {
       },
     };
 
+    // The "Token Count" / "Show Cost ($)" header toggles were removed from the
+    // UI, but their features still read settings.showTokenCount / settings.showCost
+    // (applyHeaderVisibilitySettings, the header cost render). saveAppSettings
+    // rebuilds `settings` fresh from the DOM (a full replacement, not a merge), so
+    // without this these keys would be DROPPED on every save and fall back to their
+    // defaults — silently re-enabling the token chip for anyone who'd turned it off,
+    // with no UI left to turn it back off. Preserve the prior stored preference.
+    if (_prev.showTokenCount !== undefined) settings.showTokenCount = _prev.showTokenCount;
+    if (_prev.showCost !== undefined) settings.showCost = _prev.showCost;
+
     // Save to localStorage
     this.saveAppSettingsToStorage(settings);
     this._updateLocalEchoState();