batch3 T4: file_ops + file_index services; UI endpoints; tools refactor

- services/file_ops.ts: shared listDir/viewFile/grep/findFiles core
- services/file_index.ts: per-project flat file list cached on mtime of
  project root + .git/HEAD + .git/index (rg --files honors .gitignore)
- services/tools.ts: tools delegate to file_ops, output format unchanged
- routes/projects.ts: GET /list_dir, /view_file, /files endpoints
- web client: api.projects.listDir/viewFile/files + mirrored types

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/api/client.ts

@@ -5,6 +5,8 @@ import type {
   Message,
   ModelInfo,
   SidebarResponse,
+  ListDirResult,
+  ViewFileResult,
 } from './types';
 
 export class ApiError extends Error {
@@ -47,6 +49,12 @@ export const api = {
       }),
     remove: (id: string) =>
       request<void>(`/api/projects/${id}`, { method: 'DELETE' }),
+    listDir: (id: string, path: string) =>
+      request<ListDirResult>(`/api/projects/${id}/list_dir?path=${encodeURIComponent(path)}`),
+    viewFile: (id: string, path: string) =>
+      request<ViewFileResult>(`/api/projects/${id}/view_file?path=${encodeURIComponent(path)}`),
+    files: (id: string) =>
+      request<{ files: string[] }>(`/api/projects/${id}/files`),
   },
 
   sessions: {