feat(KB-157): add soft-pause (enginePaused) alongside hard-stop (globalPause)

- Add enginePaused field to Settings type with soft-pause semantics (drain queue, don't kill agents)
- Gate scheduler, triage, auto-merge, and periodic merge retry on enginePaused
- Add Pause and Stop buttons to dashboard Header replacing single toggle
- Wire enginePaused state through App with optimistic toggle and unpause resume logic
- Add tests for scheduler, triage, dashboard CLI, Header, and App covering pause behavior

packages/cli/src/commands/__tests__/dashboard.test.ts

@@ -76,6 +76,8 @@ vi.mock("@kb/engine", async (importOriginal) => {
       stop: vi.fn(),
     })),
     aiMergeTask: vi.fn().mockResolvedValue({ merged: true }),
+    scanIdleWorktrees: vi.fn().mockResolvedValue([]),
+    cleanupOrphanedWorktrees: vi.fn().mockResolvedValue(0),
   };
 });