refactor: extract useConflictFlow, useAppSave, useVaultBridge from App.tsx

App.tsx was 702 lines and the highest-churn file (102 commits/month).
Extract three hooks to reduce it to 537 lines and distribute future
changes across focused modules:
- useConflictFlow: conflict resolution orchestration
- useAppSave: save/flush/rename orchestration
- useVaultBridge: agent/MCP file operation handlers

All files score 10.0 on CodeScene. 2226 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useAppSave.test.ts

@@ -0,0 +1,94 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { renderHook, act } from '@testing-library/react'
+import { useAppSave } from './useAppSave'
+import type { VaultEntry } from '../types'
+
+vi.mock('@tauri-apps/api/core', () => ({
+  invoke: vi.fn().mockResolvedValue(undefined),
+}))
+vi.mock('../mock-tauri', () => ({
+  isTauri: () => false,
+  mockInvoke: vi.fn().mockResolvedValue(undefined),
+}))
+
+function makeEntry(path: string, title = 'Test', filename = 'test.md'): VaultEntry {
+  return { path, title, filename, content: '', outgoingLinks: [], snippet: '', wordCount: 0, isA: 'Note', status: null, createdAt: null, modifiedAt: null, icon: null, tags: [] } as unknown as VaultEntry
+}
+
+describe('useAppSave', () => {
+  const deps = {
+    updateEntry: vi.fn(),
+    setTabs: vi.fn(),
+    setToastMessage: vi.fn(),
+    loadModifiedFiles: vi.fn(),
+    clearUnsaved: vi.fn(),
+    unsavedPaths: new Set<string>(),
+    tabs: [] as Array<{ entry: VaultEntry; content: string }>,
+    activeTabPath: null as string | null,
+    handleRenameNote: vi.fn().mockResolvedValue(undefined),
+    replaceEntry: vi.fn(),
+    resolvedPath: '/vault',
+  }
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+    deps.unsavedPaths = new Set()
+    deps.tabs = []
+    deps.activeTabPath = null
+    deps.handleRenameNote.mockResolvedValue(undefined)
+  })
+
+  function renderSave(overrides = {}) {
+    return renderHook(() => useAppSave({ ...deps, ...overrides }))
+  }
+
+  it('exposes contentChangeRef', () => {
+    const { result } = renderSave()
+    expect(result.current.contentChangeRef).toBeDefined()
+    expect(typeof result.current.contentChangeRef.current).toBe('function')
+  })
+
+  it('exposes handleSave', () => {
+    const { result } = renderSave()
+    expect(typeof result.current.handleSave).toBe('function')
+  })
+
+  it('exposes handleTitleSync', () => {
+    const { result } = renderSave()
+    expect(typeof result.current.handleTitleSync).toBe('function')
+  })
+
+  it('exposes flushBeforeAction', () => {
+    const { result } = renderSave()
+    expect(typeof result.current.flushBeforeAction).toBe('function')
+  })
+
+  it('handleSave calls save with no fallback when no active tab', async () => {
+    const { result } = renderSave()
+
+    await act(async () => { await result.current.handleSave() })
+
+    // Should not throw — just a no-op save
+  })
+
+  it('handleSave provides fallback for unsaved active tab', async () => {
+    const entry = makeEntry('/vault/note.md', 'note', 'note.md')
+    const unsavedPaths = new Set(['/vault/note.md'])
+    const tabs = [{ entry, content: '# Hello' }]
+
+    const { result } = renderSave({
+      tabs,
+      activeTabPath: '/vault/note.md',
+      unsavedPaths,
+    })
+
+    await act(async () => { await result.current.handleSave() })
+
+    // Should complete without error
+  })
+
+  it('handleContentChange is a function', () => {
+    const { result } = renderSave()
+    expect(typeof result.current.handleContentChange).toBe('function')
+  })
+})