refactor(plugins): migrate claude-timeline to scope-based lifecycle

The 12-resource hand-written destroy() is gone: every timer, listener,
observer, DOM node, and the preview panel's destroy() chain now registers
on the PluginScope and unwinds automatically. Two behaviors this adds:

- data-gv-claude-turn-id stamps on Claude's own turn nodes are rolled
  back on unmount (previously leaked).
- coachmarks accept an AbortSignal; a disposed scope closes an in-flight
  guide as 'skipped' without burning its once-per-user seen state.

Startup registers as a pending scope effect, so unmounting mid-start
barriers on it instead of relying on a destroyed flag. updateSettings
stays as the fine-grained path — grow-only virtualized markers and the
rail DOM survive settings changes, unchanged. One-shot scope timers and
frames now release their ledger slot on firing, keeping debounce-heavy
consumers bounded. The 641-line behavior suite passes with only the
stop-is-now-async await adjustments.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Nagi-ovo

src/features/plugins/builtin/claudeTimeline/index.test.ts

@@ -100,8 +100,8 @@ describe('Claude timeline', () => {
     Object.defineProperty(window, 'scrollY', { configurable: true, value: 0 });
   });
 
-  afterEach(() => {
-    stopClaudeTimeline();
+  afterEach(async () => {
+    await stopClaudeTimeline();
     vi.useRealTimers();
   });
 
@@ -627,13 +627,23 @@ describe('Claude timeline', () => {
     queryAll.mockRestore();
   });
 
+  it('rolls back data-gv-claude-turn-id stamps on stop', async () => {
+    addTurn('first prompt');
+    startClaudeTimeline();
+    await flush();
+    expect(document.querySelectorAll('[data-gv-claude-turn-id]').length).toBeGreaterThan(0);
+
+    await stopClaudeTimeline();
+    expect(document.querySelectorAll('[data-gv-claude-turn-id]').length).toBe(0);
+  });
+
   it('removes UI on stop', async () => {
     addTurn('first prompt');
     startClaudeTimeline();
     await flush();
     expect(document.querySelector('.gemini-timeline-bar')).toBeTruthy();
 
-    stopClaudeTimeline();
+    await stopClaudeTimeline();
     expect(document.querySelector('.gemini-timeline-bar')).toBeNull();
     expect(document.querySelector('.timeline-preview-toggle')).toBeNull();
     expect(document.querySelector('#claude-timeline-tooltip')).toBeNull();