feat(nudges): Phase 6 — Margot soft-touch nudges via frontend nudge bus

Frontend-owned nudge bus that consumes in-app signals Corbie already
produces, applies suppression, and fans out to OS notification + an
optional TTS read-aloud. OS-wide keyboard/window activity detection
stays deferred per the revised roadmap — the plan before rewrite
would have been brittle on Wayland, permission-heavy on macOS, and
low-quality everywhere.

Triggers (v1, all in-app signals):
- inactivity_with_active_timer — timer running, window blurred ≥ 90 s,
  at least 60 s into the timer.
- pending_morning_triage — ritual enabled, past 10:00 local, last
  shown ≠ today. Polls every 5 min while focused.
- micro_step_idle — micro-step decomposition created, no child step
  or parent task completed within 15 min.

Suppression:
- Respects nudgesEnabled + nudgesMuted.
- No nudge while the app has focus (document.hasFocus).
- Hard cap 3 per rolling hour.
- Permission requested via @tauri-apps/plugin-notification on first
  delivery; denial is silently respected.

Rust side:
- tauri-plugin-notification registered + ACL entries on the main-
  window capability only (secondary windows can't fire nudges).
- commands::nudges::deliver_nudge — thin wrapper, security-guarded
  via ensure_main_window, delegates to the plugin. No DB writes —
  the roadmap's nudges-audit table is deferred until a concrete need
  emerges.

Frontend glue:
- nudgeBus.svelte.ts — subscribes to window events, applies
  suppression, calls deliver_nudge (+ tts_speak when speakAloud is
  on).
- kon:task-completed now dispatched on complete_task_cmd success.
- kon:microstep-generated + kon:step-completed dispatched from
  MicroSteps so the idle trigger can clear itself on any engagement.
- kon:focus-timer-cancelled added to focusTimer so the bus can reset
  its inactivity state on cancel, not only on natural completion.
- nudgeBus started from +layout.svelte onMount, stopped on destroy.

Settings:
- New "Nudges" section with three toggles: Enable nudges, Mute for
  now (separate so a hard mute doesn't lose preferences),
  Speak nudges aloud (reuses Phase 4 TTS).
- All default OFF. No first-run prompt — nudges are a Settings-found
  feature rather than a walkthrough step.

Out of scope per the revised Phase 6 spec: OS-wide keyboard/window
hooks, biometric signals, custom trigger editor (Phase 7), notification
sound (platform variance too high for Layer-1 — revisit in Phase 9
polish with a bundled .wav).

Gates: fmt clean, clippy -D warnings clean, cargo test 262/0 (4
existing + 262 current), npm run check 0/0, npm run build green.

src/routes/+layout.svelte

@@ -25,6 +25,7 @@
   import { toasts } from "$lib/stores/toasts.svelte.js";
   import { initI18n } from "$lib/i18n";
   import { refreshLlmStatus } from "$lib/stores/llmStatus.svelte.js";
+  import { startNudgeBus, stopNudgeBus } from "$lib/stores/nudgeBus.svelte.ts";
 
   import { page as sveltePage } from "$app/stores";
 
@@ -277,6 +278,12 @@
     // Phase 5: subscribe to tray wind-down event.
     setupWindDownListener();
 
+    // Phase 6: start the nudge bus. The bus is self-gated on
+    // settings.nudgesEnabled internally — starting it here is cheap
+    // (just event listeners + two intervals), and means users don't
+    // need to restart after flipping the toggle.
+    startNudgeBus();
+
     // Diagnostics: capture every uncaught frontend error to error_log.
     installGlobalErrorCapture();
 
@@ -384,6 +391,7 @@
     if (unlistenWindDown) {
       unlistenWindDown();
     }
+    stopNudgeBus();
   });
 </script>