feat(mcp): detect borrowed git worktree index and surface on read tools (#312)

When a worktree is nested inside the main checkout (e.g. agent tools that place
worktrees under .claude/worktrees/<name>/), the nearest-.codegraph walk resolves
UP to the main checkout's index and queries silently return that tree's code —
usually a different branch. Symbols changed only in the worktree are invisible,
and nothing tells the user (#155).

Two layers:

- **Detection** (src/sync/worktree.ts): detectWorktreeIndexMismatch() compares
  the caller's git working-tree root vs the resolved index root via
  'git rev-parse --show-toplevel'. Best-effort; no git / not a repo / monorepo
  subdir / plain-ancestor index → no warning.
- **Surface**: codegraph status (CLI + MCP) embeds a verbose multi-line warning;
  every MCP read tool (search/context/trace/callers/callees/impact/explore/node/
  files) prefixes a compact one-line notice naming the borrowed index and the
  fix (codegraph init -i in the worktree). Detection is cached per session per
  start path, so it costs at most a single pair of 'git rev-parse' spawns per
  project no matter how many tool calls — respects the wall-clock-latency
  invariant.

Real-git tests (no mocking) cover both layers. Validated on macOS / Linux
(Docker) / Windows (Parallels VM); 11/11 worktree tests green on all three.

Closes #155

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

src/sync/index.ts

@@ -8,6 +8,7 @@
  * - FileWatcher: Debounced fs.watch that auto-triggers sync on file changes
  * - Watch policy: decides when the watcher must be disabled (e.g. WSL2 /mnt)
  * - Git sync hooks: opt-in commit/merge/checkout hooks when watching is off
+ * - Git worktree awareness: detect when a query borrows another tree's index
  * - Content hashing for change detection (in extraction module)
  * - Incremental reindexing (in extraction module)
  */
@@ -23,3 +24,10 @@ export {
   type GitHookName,
   type GitHookResult,
 } from './git-hooks';
+export {
+  gitWorktreeRoot,
+  detectWorktreeIndexMismatch,
+  worktreeMismatchWarning,
+  worktreeMismatchNotice,
+  type WorktreeIndexMismatch,
+} from './worktree';