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

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

extension/src/service-worker/github.ts

@@ -14,12 +14,14 @@ import { uint8ArrayToBase64, base64ToUint8Array, BLOB_THRESHOLD_BYTES } from './
 export class GitHubHost implements GitHost {
   private baseUrl: string;
   private gitApiBase: string;
+  private commitsUrl: string;
   private branch: string = 'main';
   private headers: Record<string, string>;
 
   constructor(repoPath: string, apiToken: string) {
     this.baseUrl = `https://api.github.com/repos/${repoPath}/contents`;
     this.gitApiBase = `https://api.github.com/repos/${repoPath}/git`;
+    this.commitsUrl = `https://api.github.com/repos/${repoPath}/commits`;
     this.headers = {
       'Authorization': `Bearer ${apiToken}`,
       'Content-Type': 'application/json',
@@ -109,6 +111,24 @@ export class GitHubHost implements GitHost {
     return json.map((item: { name: string }) => item.name);
   }
 
+  async lastCommit(path: string): Promise<{ sha: string; author: string; date: string } | null> {
+    try {
+      const url = `${this.commitsUrl}?path=${encodeURIComponent(path)}&per_page=1`;
+      const resp = await fetch(url, { headers: this.headers });
+      if (!resp.ok) return null;
+      const json = await resp.json();
+      if (!Array.isArray(json) || json.length === 0) return null;
+      const c = json[0];
+      return {
+        sha: String(c.sha).slice(0, 7),
+        author: c.commit?.author?.name ?? 'unknown',
+        date: c.commit?.author?.date ?? '',
+      };
+    } catch {
+      return null;
+    }
+  }
+
   async putBlob(path: string, content: Uint8Array, message: string): Promise<string> {
     if (content.length <= BLOB_THRESHOLD_BYTES) {
       // Contents API path — same as writeFile