feat(ext/sw): GitHost.lastCommit() for vault-presence metadata

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

extension/src/service-worker/git-host.ts

@@ -17,6 +17,11 @@ export interface GitHost {
   /// List file names in a directory (non-recursive).
   listDir(path: string): Promise<string[]>;
 
+  /// Best-effort: returns metadata for the most recent commit touching `path`.
+  /// Returns null if the path has no commits, the API fails, or the host
+  /// doesn't support the lookup. Callers must tolerate null.
+  lastCommit(path: string): Promise<{ sha: string; author: string; date: string } | null>;
+
   /// Write an opaque binary blob to the repo. Optimized for large
   /// attachments — implementations switch from Contents API to Git
   /// Data API when content exceeds BLOB_THRESHOLD_BYTES.