Handle lightweight tags in get_tag

[kerobbi]

Summary

Add lightweight tag handling to get_tag by returning the ref directly when the tag points to a commit instead of trying to fetch a tag object that doesn't exist.

Why

Fixes #2297

get_tag does two API calls: resolve the ref, then fetch the tag object. However, while this is fine for annotated tags, lightweight ones point directly to a commit and have no tag object, so the second call always returns a 404.

What changed

• Added a type check after resolving the tag ref (if the tag is lightweight, return the ref directly instead of attempting to fetch a non-existent tag object)
• Updated existing tests and added test case for lightweight tag retrieval

MCP impact

• No tool or API changes
• Tool schema or behavior changed - get_tag now handles lightweight tags instead of returning a 404
• New tool added

Prompts tested (tool changes only)

• "Get tag X from Z repo" - annotated works as before
• "Get tag Y from Z repo" - lightweight now returns ref data instead of a 404

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

Adds support for lightweight tags in the get_tag tool by returning the resolved tag ref directly when the ref points to a commit (instead of attempting to fetch a non-existent tag object and returning 404s).

Changes:

• Added a lightweight-vs-annotated tag type check after resolving the tag ref in get_tag.
• Updated Test_GetTag to cover both annotated tag (tag object) and lightweight tag (ref-only) responses.

Show a summary per file

File	Description
pkg/github/repositories.go	Adds early return path for lightweight tags based on resolved ref object type.
pkg/github/repositories_test.go	Extends get_tag tests to validate lightweight tag behavior and adjusts assertions accordingly.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2