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

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

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

@@ -11,6 +11,11 @@ export interface GitHost {
   /// Create or update a file in the repo with a commit message.
   writeFile(path: string, content: Uint8Array, message: string): Promise<void>;
 
+  /// Like writeFile, but throws if the file already exists. Used by setup
+  /// wizard to refuse to clobber existing vault state. Implementation must
+  /// pre-check existence and only POST/PUT-create — never include a sha.
+  writeFileCreateOnly(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>;