feat(ext/sw): GitHubHost.putBlob with Git Data API fallback

Blobs ≤ BLOB_THRESHOLD_BYTES (900 KB) take the Contents API path
(same as writeFile). Larger blobs use the Git Data API: POST blob,
GET ref + commit, POST tree (with base_tree), POST commit, PATCH ref.
Tests cover both paths plus error propagation.

getBlob/deleteBlob are thin wrappers over readFile/deleteFile.

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

extension/src/service-worker/github.ts

@@ -1,5 +1,5 @@
 import type { GitHost } from './git-host';
-import { uint8ArrayToBase64, base64ToUint8Array } from './git-host';
+import { uint8ArrayToBase64, base64ToUint8Array, BLOB_THRESHOLD_BYTES } from './git-host';
 
 /// GitHub Contents API implementation.
 ///
@@ -13,10 +13,13 @@ import { uint8ArrayToBase64, base64ToUint8Array } from './git-host';
 
 export class GitHubHost implements GitHost {
   private baseUrl: string;
+  private gitApiBase: 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.headers = {
       'Authorization': `Bearer ${apiToken}`,
       'Content-Type': 'application/json',
@@ -106,8 +109,86 @@ export class GitHubHost implements GitHost {
     return json.map((item: { name: string }) => item.name);
   }
 
-  async putBlob(_path: string, _content: Uint8Array, _message: string): Promise<string> {
-    throw new Error(`GitHubHost.putBlob not implemented — Task 3`);
+  async putBlob(path: string, content: Uint8Array, message: string): Promise<string> {
+    if (content.length <= BLOB_THRESHOLD_BYTES) {
+      // Contents API path — same as writeFile
+      await this.writeFile(path, content, message);
+      return path;
+    }
+
+    // Git Data API fallback for large blobs.
+    // 1. Create the blob.
+    const blobResp = await fetch(`${this.gitApiBase}/blobs`, {
+      method: 'POST',
+      headers: this.headers,
+      body: JSON.stringify({
+        content: uint8ArrayToBase64(content),
+        encoding: 'base64',
+      }),
+    });
+    if (!blobResp.ok) {
+      const text = await blobResp.text();
+      throw new Error(`GitHub putBlob create-blob ${path}: ${blobResp.status} ${text}`);
+    }
+    const { sha: blobSha } = await blobResp.json() as { sha: string };
+
+    // 2. Get current ref (branch tip commit SHA).
+    const refResp = await fetch(`${this.gitApiBase}/refs/heads/${this.branch}`, { headers: this.headers });
+    if (!refResp.ok) {
+      throw new Error(`GitHub putBlob get-ref ${this.branch}: ${refResp.status} ${refResp.statusText}`);
+    }
+    const { object: { sha: commitSha } } = await refResp.json() as { object: { sha: string } };
+
+    // 3. Get current commit's tree SHA.
+    const commitResp = await fetch(`${this.gitApiBase}/commits/${commitSha}`, { headers: this.headers });
+    if (!commitResp.ok) {
+      throw new Error(`GitHub putBlob get-commit ${commitSha}: ${commitResp.status} ${commitResp.statusText}`);
+    }
+    const { tree: { sha: baseTreeSha } } = await commitResp.json() as { tree: { sha: string } };
+
+    // 4. Create new tree adding the blob at `path` on top of base tree.
+    const treeResp = await fetch(`${this.gitApiBase}/trees`, {
+      method: 'POST',
+      headers: this.headers,
+      body: JSON.stringify({
+        base_tree: baseTreeSha,
+        tree: [{ path, mode: '100644', type: 'blob', sha: blobSha }],
+      }),
+    });
+    if (!treeResp.ok) {
+      const text = await treeResp.text();
+      throw new Error(`GitHub putBlob create-tree: ${treeResp.status} ${text}`);
+    }
+    const { sha: newTreeSha } = await treeResp.json() as { sha: string };
+
+    // 5. Create new commit pointing at the new tree.
+    const newCommitResp = await fetch(`${this.gitApiBase}/commits`, {
+      method: 'POST',
+      headers: this.headers,
+      body: JSON.stringify({
+        message,
+        tree: newTreeSha,
+        parents: [commitSha],
+      }),
+    });
+    if (!newCommitResp.ok) {
+      const text = await newCommitResp.text();
+      throw new Error(`GitHub putBlob create-commit: ${newCommitResp.status} ${text}`);
+    }
+    const { sha: newCommitSha } = await newCommitResp.json() as { sha: string };
+
+    // 6. Fast-forward branch to the new commit.
+    const updateRefResp = await fetch(`${this.gitApiBase}/refs/heads/${this.branch}`, {
+      method: 'PATCH',
+      headers: this.headers,
+      body: JSON.stringify({ sha: newCommitSha, force: false }),
+    });
+    if (!updateRefResp.ok) {
+      const text = await updateRefResp.text();
+      throw new Error(`GitHub putBlob update-ref: ${updateRefResp.status} ${text}`);
+    }
+
+    return path;
   }
 
   async getBlob(path: string): Promise<Uint8Array> {