fix(statusline): GET /api/settings never writes, and a save sends the collection switch only on a flip

Two follow-ups to #361's sticky telemetry switch.

GET /api/settings reconciled an absent showPlanUsageLimits by persisting
true, but readJsonConfig() answers {} for ANY read failure (a parse
error, EACCES, EMFILE, a read landing inside PUT's non-atomic write), not
only ENOENT, and every page load calls this route, so one unlucky read
replaced the whole settings file with a one-key file. The route is a
plain read again and the default moved into the reader:
readPlanUsageTelemetryEnabled() treats an absent key as ON, the same way
readWorkspaceHooksEnabled() does, which is what the desktop chip already
shows for an install that never touched the setting.

saveAppSettings() sent showPlanUsageLimits on every save. The chip
defaults OFF on handhelds, so a phone saving its font size persisted
false and switched collection off for every desktop, whose chip then
went stale with no error anywhere. The key is now stripped like the
other per-device display keys and re-added only when the save FLIPS the
chip relative to what the device had (planUsageCollectionFlip), so an
explicit toggle on any device still writes it in either direction.

Tests pin both: the GET route with a mocked filesystem (absent, missing,
EACCES, garbage, explicit), the reader default, and the flip helper plus
its wiring in saveAppSettings.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/hooks-config.ts

@@ -1044,10 +1044,21 @@ export async function ensureStatusLineExporterScript(): Promise<string> {
  * to thread a request-time flag through: they all already construct a
  * session via TmuxManager.createSession/respawnPane, which reads this at
  * spawn time.
+ *
+ * An ABSENT key means ON, mirroring readWorkspaceHooksEnabled() above: the
+ * client shows the chip and its checkbox as already on for a desktop that has
+ * never touched the setting (planUsageChipEnabled() in settings-ui.js), and
+ * the exporter only ever posts to THIS Codeman over loopback, so the honest
+ * default for an install that never said otherwise is the one the user can
+ * see. Resolving the default here, in the reader, is what lets
+ * `GET /api/settings` stay a plain read: a reconcile write there ran on every
+ * page load and could replace an unreadable settings.json with a one-key
+ * file. Only an explicit `false` (a save that flipped the chip off on some
+ * device) turns collection off.
  */
 export async function readPlanUsageTelemetryEnabled(): Promise<boolean> {
   const settings = await readJsonConfig<Record<string, unknown>>(SETTINGS_PATH, 'settings.json', {});
-  return settings.showPlanUsageLimits === true;
+  return settings.showPlanUsageLimits !== false;
 }
 
 /**