feat: add git API layer with Gitea and GitHub implementations

GitHost interface for reading/writing vault files via REST API.
Gitea and GitHub implementations handle base64 content encoding,
SHA-based updates, and directory listing.

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

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

@@ -0,0 +1,54 @@
+/// Abstract interface for reading/writing vault files on a git host.
+///
+/// Both Gitea and GitHub expose a "repo contents" REST API that lets us
+/// read, write, and delete individual files without cloning the repo.
+/// This interface captures just the operations the vault needs.
+
+export interface GitHost {
+  /// Read a single file from the repo, returning its raw bytes.
+  readFile(path: string): Promise<Uint8Array>;
+
+  /// Create or update a file in the repo with a commit message.
+  writeFile(path: string, content: Uint8Array, message: string): Promise<void>;
+
+  /// Delete a file from the repo with a commit message.
+  deleteFile(path: string, message: string): Promise<void>;
+
+  /// List file names in a directory (non-recursive).
+  listDir(path: string): Promise<string[]>;
+}
+
+/// Convert a Uint8Array to a base64 string (works in service worker context).
+export function uint8ArrayToBase64(bytes: Uint8Array): string {
+  let binary = '';
+  for (let i = 0; i < bytes.length; i++) {
+    binary += String.fromCharCode(bytes[i]);
+  }
+  return btoa(binary);
+}
+
+/// Convert a base64 string to a Uint8Array.
+export function base64ToUint8Array(base64: string): Uint8Array {
+  const binary = atob(base64);
+  const bytes = new Uint8Array(binary.length);
+  for (let i = 0; i < binary.length; i++) {
+    bytes[i] = binary.charCodeAt(i);
+  }
+  return bytes;
+}
+
+/// Factory function that returns the appropriate GitHost implementation.
+import { GiteaHost } from './gitea';
+import { GitHubHost } from './github';
+
+export function createGitHost(
+  hostType: 'gitea' | 'github',
+  hostUrl: string,
+  repoPath: string,
+  apiToken: string,
+): GitHost {
+  if (hostType === 'gitea') {
+    return new GiteaHost(hostUrl, repoPath, apiToken);
+  }
+  return new GitHubHost(repoPath, apiToken);
+}