feat: Phase 2 — store audit, migration clusters, ADR, settings domain migration

- MIGRATION_CLUSTERS.md: reactive dependency graph across 20 bridges/stores
- PHASE1_STORE_AUDIT.md: 11 stores audited (3 clean, 5 bridge-dependent, 3 platform-specific)
- ADR-001: dual-stack binding strategy (accepted, S-1+S-3 hybrid)
- Settings domain migration: all 6 settings components + App.svelte + FilesTab
  migrated from settings-bridge to getBackend() calls

src/App.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import { initTheme } from './lib/stores/theme.svelte';
-  import { getSetting } from './lib/adapters/settings-bridge';
+  import { getSetting } from './lib/stores/settings-store.svelte';
   import { isDetachedMode, getDetachedConfig } from './lib/utils/detach';
   import { startAgentDispatcher, stopAgentDispatcher } from './lib/agent-dispatcher';
   import { startHealthTick, stopHealthTick, clearHealthTracking } from './lib/stores/health.svelte';