feat(ext/sw): extend GitHost interface with putBlob/getBlob/deleteBlob

Adds the three blob ops to the interface and a BLOB_THRESHOLD_BYTES
constant. Both GitHubHost and GiteaHost ship temporary stubs so the
build stays green until tasks 3-4 fill in real implementations.

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

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

@@ -16,8 +16,28 @@ export interface GitHost {
 
   /// List file names in a directory (non-recursive).
   listDir(path: string): Promise<string[]>;
+
+  /// Write an opaque binary blob to the repo. Optimized for large
+  /// attachments — implementations switch from Contents API to Git
+  /// Data API when content exceeds BLOB_THRESHOLD_BYTES.
+  /// Returns the path that was written (same as input, for chaining).
+  putBlob(path: string, content: Uint8Array, message: string): Promise<string>;
+
+  /// Read an opaque binary blob from the repo. Same semantics as
+  /// readFile for current sizes. Distinct method so we can later add
+  /// streaming/chunked reads for very large blobs.
+  getBlob(path: string): Promise<Uint8Array>;
+
+  /// Delete a blob from the repo. Currently identical to deleteFile;
+  /// kept distinct for symmetry with putBlob.
+  deleteBlob(path: string, message: string): Promise<void>;
 }
 
+/// Pre-base64 byte size at which putBlob switches from Contents API to
+/// Git Data API. 900 KB leaves headroom for base64 inflation (1.33×) and
+/// JSON wrapping under both GitHub's and Gitea's Contents API soft-limits.
+export const BLOB_THRESHOLD_BYTES = 900 * 1024;
+
 /// Convert a Uint8Array to a base64 string (works in service worker context).
 export function uint8ArrayToBase64(bytes: Uint8Array): string {
   let binary = '';