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>

test/usage-telemetry.test.ts

@@ -68,6 +68,24 @@ describe('parseStatusTelemetry', () => {
   it('ignores a zero/negative reset timestamp', () => {
     expect(parseStatusTelemetry({ rate_limits: { five_hour: { used_percentage: 10, resets_at: 0 } } })).toBeNull();
   });
+
+  it('keeps a NaN percentage as 0 and drops a window with a non-finite reset', () => {
+    const t = parseStatusTelemetry({
+      rate_limits: {
+        five_hour: { used_percentage: NaN, resets_at: 1781409000 },
+        seven_day: { used_percentage: 40, resets_at: Infinity },
+      },
+    });
+    expect(t!.fiveHour).toEqual({ usedPercentage: 0, resetAt: 1781409000 * 1000 });
+    expect(t!.sevenDay).toBeUndefined();
+  });
+
+  it('rounds a fractional resets_at to whole milliseconds', () => {
+    const t = parseStatusTelemetry({
+      rate_limits: { five_hour: { used_percentage: 10, resets_at: 1781409000.7 } },
+    });
+    expect(t!.fiveHour?.resetAt).toBe(Math.round(1781409000.7 * 1000));
+  });
 });
 
 describe('parseSessionStatus', () => {
@@ -118,4 +136,27 @@ describe('telemetrySignature', () => {
     })!;
     expect(telemetrySignature(moved)).not.toBe(telemetrySignature(a));
   });
+
+  it('ignores contextUsedPercentage (not displayed) so it does not churn each message', () => {
+    const base = { rate_limits: { five_hour: { used_percentage: 15, resets_at: 1781409000 } } };
+    const a = parseStatusTelemetry({ ...base, context_window: { used_percentage: 56 } })!;
+    const b = parseStatusTelemetry({ ...base, context_window: { used_percentage: 91 } })!;
+    expect(telemetrySignature(a)).toBe(telemetrySignature(b));
+  });
+
+  it('keys on the ROUNDED window percentage (matches the chip) — sub-integer drift is ignored', () => {
+    const sig = (p: number) =>
+      telemetrySignature(
+        parseStatusTelemetry({ rate_limits: { five_hour: { used_percentage: p, resets_at: 1781409000 } } })!
+      );
+    expect(sig(15.1)).toBe(sig(15.4)); // both render as 15%
+    expect(sig(15.1)).not.toBe(sig(15.6)); // 15% vs 16%
+  });
+
+  it('excludes cost/model (not shown in the chip) from the signature', () => {
+    const base = { rate_limits: { five_hour: { used_percentage: 15, resets_at: 1781409000 } } };
+    const a = parseStatusTelemetry({ ...base, cost: { total_cost_usd: 0.01 }, model: { display_name: 'A' } })!;
+    const b = parseStatusTelemetry({ ...base, cost: { total_cost_usd: 9.99 }, model: { display_name: 'B' } })!;
+    expect(telemetrySignature(a)).toBe(telemetrySignature(b));
+  });
 });