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/app.js

@@ -1817,7 +1817,6 @@ class CodemanApp {
   // Claude plan usage limits (5-hour + weekly) — account-global, so the latest
   // sample from any session drives the shared header chip.
   _onSessionStatusTelemetry(data) {
-    this._latestPlanUsage = data;
     this.updatePlanUsageChip(data);
     // Persist last-known so the chip shows immediately on the next page load /
     // SSE reconnect, instead of staying blank until a session next renders.
@@ -1848,10 +1847,17 @@ class CodemanApp {
     if (five === null && seven === null) return;
     // Per-window color by how much is used up: green < 60%, yellow 60–84%, red ≥ 85%.
     const colorClass = (p) => (p >= 85 ? 'pu-red' : p >= 60 ? 'pu-yellow' : 'pu-green');
-    const seg = (label, p) =>
-      p === null
-        ? ''
-        : `<span class="pu-win"><span class="pu-label">${label}</span><span class="pu-val ${colorClass(p)}">${p}%</span></span>`;
+    // innerHTML here is XSS-safe ONLY because every interpolated value is a
+    // coerced finite number and the labels/classes are fixed literals. If a
+    // string field (e.g. modelDisplayName, which the route also broadcasts) is
+    // ever shown in this chip, render it via textContent — never interpolate an
+    // untrusted string into this template.
+    const seg = (label, p) => {
+      if (p === null) return '';
+      const n = Math.round(Number(p));
+      if (!Number.isFinite(n)) return '';
+      return `<span class="pu-win"><span class="pu-label">${label}</span><span class="pu-val ${colorClass(n)}">${n}%</span></span>`;
+    };
     chip.innerHTML = [seg('5h', five), seg('7d', seven)].filter(Boolean).join('<span class="pu-sep">·</span>');
     const resetStr = (w) => (w && w.resetAt ? new Date(w.resetAt).toLocaleString() : '—');
     chip.title =