Problem

Ref.create_tag creates lightweight tags only (a ref pointing directly at a commit SHA). Annotated tags require manually writing a Tag object and then creating the ref — there’s no convenience API.

Impact

• Common git workflow (signed/annotated releases) requires low-level object manipulation
• API inconsistency: tags are a first-class concept but only half-supported

Acceptance Criteria

• Ref.create_annotated_tag(repo, name, target_sha, tagger, message) API
• Writes a Tag object and creates the ref atomically
• Supports optional GPG signature field
• Returns {:ok, tag_sha} on success
• Interop test: annotated tag readable by git tag -v