Add exact temporary tag checkout

[andrew]

Add CheckoutTag for temporarily inspecting an exact local tag without exposing callers to Git ref parsing. It peels annotated tags, checks out a validated commit SHA, and returns a context-aware restore function that preserves attached or detached HEAD state.

Expose ErrTagNotFound so callers can try alternate exact tag names without hiding other repository errors.

[Copilot]

Pull request overview

This PR introduces a new CheckoutTag helper to temporarily inspect the commit behind an exact local tag (including peeling annotated tags), returning a restore function that attempts to reattach the prior branch when safe. It also exposes ErrTagNotFound so callers can distinguish “tag absent” from other repository errors without implementing Git ref parsing themselves.

Changes:

• Add CheckoutTag (plus helpers) to validate, resolve, peel, and detach-checkout an exact local tag, and to restore prior HEAD/branch state.
• Add ErrTagNotFound for callers to errors.Is against when trying alternate tag spellings.
• Add documentation and a comprehensive test suite covering annotated tags, detached-head restore, moved-branch refusal, and context cancellation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
README.md	Documents the new CheckoutTag API with an example and ErrTagNotFound guidance.
doc.go	Updates package overview to mention exact temporary tag checkouts.
checkout.go	Implements CheckoutTag, ErrTagNotFound, and supporting Git resolution/checkout helpers.
checkout_test.go	Adds tests for exact tag resolution, annotated tag peeling, restore behavior, and context handling.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.