feat(ext/sw): GitHost.writeFileCreateOnly() refuses to overwrite

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

extension/src/service-worker/gitea.ts

@@ -82,6 +82,23 @@ export class GiteaHost implements GitHost {
     }
   }
 
+  async writeFileCreateOnly(path: string, content: Uint8Array, message: string): Promise<void> {
+    const existing = await fetch(`${this.baseUrl}/${path}`, { headers: this.headers });
+    if (existing.ok) {
+      throw new Error(`writeFileCreateOnly: ${path} already exists`);
+    }
+    const b64 = uint8ArrayToBase64(content);
+    const resp = await fetch(`${this.baseUrl}/${path}`, {
+      method: 'POST',
+      headers: this.headers,
+      body: JSON.stringify({ content: b64, message }),
+    });
+    if (!resp.ok) {
+      const text = await resp.text();
+      throw new Error(`Gitea writeFileCreateOnly ${path}: ${resp.status} ${text}`);
+    }
+  }
+
   async deleteFile(path: string, message: string): Promise<void> {
     // Need the current SHA to delete.
     const existing = await fetch(`${this.baseUrl}/${path}`, {