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/github.ts

@@ -0,0 +1,108 @@
+import type { GitHost } from './git-host';
+import { uint8ArrayToBase64, base64ToUint8Array } from './git-host';
+
+/// GitHub Contents API implementation.
+///
+/// Endpoints:
+///   GET    https://api.github.com/repos/{repoPath}/contents/{path}
+///   PUT    https://api.github.com/repos/{repoPath}/contents/{path}   (create/update)
+///   DELETE https://api.github.com/repos/{repoPath}/contents/{path}
+///
+/// Auth: `Bearer {apiToken}` header.
+/// Content is base64-encoded. Updates and deletes require the current file SHA.
+
+export class GitHubHost implements GitHost {
+  private baseUrl: string;
+  private headers: Record<string, string>;
+
+  constructor(repoPath: string, apiToken: string) {
+    this.baseUrl = `https://api.github.com/repos/${repoPath}/contents`;
+    this.headers = {
+      'Authorization': `Bearer ${apiToken}`,
+      'Content-Type': 'application/json',
+      'Accept': 'application/vnd.github.v3+json',
+      'X-GitHub-Api-Version': '2022-11-28',
+    };
+  }
+
+  async readFile(path: string): Promise<Uint8Array> {
+    const resp = await fetch(`${this.baseUrl}/${path}`, {
+      headers: this.headers,
+    });
+    if (!resp.ok) {
+      throw new Error(`GitHub readFile ${path}: ${resp.status} ${resp.statusText}`);
+    }
+    const json = await resp.json();
+    const clean = (json.content as string).replace(/\n/g, '');
+    return base64ToUint8Array(clean);
+  }
+
+  async writeFile(path: string, content: Uint8Array, message: string): Promise<void> {
+    const b64 = uint8ArrayToBase64(content);
+
+    // Try to get the current SHA for an update; if 404 it's a create.
+    let sha: string | null = null;
+    try {
+      const existing = await fetch(`${this.baseUrl}/${path}`, {
+        headers: this.headers,
+      });
+      if (existing.ok) {
+        const json = await existing.json();
+        sha = json.sha as string;
+      }
+    } catch {
+      // File does not exist — will create.
+    }
+
+    const body: Record<string, unknown> = { content: b64, message };
+    if (sha) {
+      body.sha = sha;
+    }
+
+    const resp = await fetch(`${this.baseUrl}/${path}`, {
+      method: 'PUT',
+      headers: this.headers,
+      body: JSON.stringify(body),
+    });
+
+    if (!resp.ok) {
+      const text = await resp.text();
+      throw new Error(`GitHub writeFile ${path}: ${resp.status} ${text}`);
+    }
+  }
+
+  async deleteFile(path: string, message: string): Promise<void> {
+    const existing = await fetch(`${this.baseUrl}/${path}`, {
+      headers: this.headers,
+    });
+    if (!existing.ok) {
+      throw new Error(`GitHub deleteFile ${path}: file not found (${existing.status})`);
+    }
+    const json = await existing.json();
+    const sha = json.sha as string;
+
+    const resp = await fetch(`${this.baseUrl}/${path}`, {
+      method: 'DELETE',
+      headers: this.headers,
+      body: JSON.stringify({ message, sha }),
+    });
+
+    if (!resp.ok) {
+      const text = await resp.text();
+      throw new Error(`GitHub deleteFile ${path}: ${resp.status} ${text}`);
+    }
+  }
+
+  async listDir(path: string): Promise<string[]> {
+    const resp = await fetch(`${this.baseUrl}/${path}`, {
+      headers: this.headers,
+    });
+    if (!resp.ok) {
+      if (resp.status === 404) return [];
+      throw new Error(`GitHub listDir ${path}: ${resp.status} ${resp.statusText}`);
+    }
+    const json = await resp.json();
+    if (!Array.isArray(json)) return [];
+    return json.map((item: { name: string }) => item.name);
+  }
+}