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/schemas.ts

@@ -195,12 +195,20 @@ export const ResizeSchema = z.object({
  * subset Codeman displays; unknown keys (session_id, transcript_path, cwd, …)
  * are stripped by z.object. Auth-exempt like /api/hook-event.
  */
+// NOTE: every modeled field is `.nullish()` (not `.optional()`) on purpose.
+// Claude's statusline blob is officially shipped but undocumented in exact
+// shape, and `z.optional()` REJECTS an explicit `null` (accepts only
+// `undefined`) — a single stray `null` (e.g. `cost:{total_cost_usd:null}`)
+// would 400 the ENTIRE POST before the deliberately-tolerant parser
+// (usage-telemetry.ts, which only acts on `typeof === 'number'/'string'`) ever
+// runs, silently killing the chip's data feed. `.nullish()` keeps the schema
+// gate as forgiving as the parser it guards.
 const RateLimitWindowSchema = z
   .object({
-    used_percentage: z.number().optional(),
-    resets_at: z.number().optional(),
+    used_percentage: z.number().nullish(),
+    resets_at: z.number().nullish(),
   })
-  .optional();
+  .nullish();
 
 export const StatusTelemetrySchema = z.object({
   sessionId: z.string().min(1).max(100),
@@ -211,18 +219,18 @@ export const StatusTelemetrySchema = z.object({
           five_hour: RateLimitWindowSchema,
           seven_day: RateLimitWindowSchema,
         })
-        .optional(),
+        .nullish(),
       context_window: z
         .object({
-          used_percentage: z.number().optional(),
-          total_input_tokens: z.number().optional(),
-          total_output_tokens: z.number().optional(),
+          used_percentage: z.number().nullish(),
+          total_input_tokens: z.number().nullish(),
+          total_output_tokens: z.number().nullish(),
         })
-        .optional(),
-      cost: z.object({ total_cost_usd: z.number().optional() }).optional(),
-      model: z.object({ display_name: z.string().max(100).optional() }).optional(),
+        .nullish(),
+      cost: z.object({ total_cost_usd: z.number().nullish() }).nullish(),
+      model: z.object({ display_name: z.string().max(100).nullish() }).nullish(),
     })
-    .optional(),
+    .nullish(),
 });
 
 // ========== Case Routes ==========