Support multi-branch CLI comparisons

[dastratakos]

Summary

Add diffity-style git ref arguments to the stagereview CLI so users can prepare and view chapter runs for branch, tag, and range comparisons.

Changes

• Added positional refs and --compare handling to prep and show.
• Resolved two-ref and range comparisons through merge-base while preserving existing working-tree scopes.
• Updated README and stage-chapters skill docs, with git-level coverage for branch comparisons.

Testing

pnpm lint; pnpm typecheck; pnpm test; pnpm build

---

[stage-review]

Ready to review this PR? Stage has broken it down into 5 individual chapters for you:

	Title
1	Build multi-ref scope resolution in git utilities
2	Thread multi-ref options through the prep command
3	Thread multi-ref options through the show command
4	Expose positional refs and --compare flag in CLI commands
5	Test multi-ref resolution and update documentation

_{Chapters generated by Stage for commit 946bddf on May 23, 2026 4:48pm UTC.}

[gemini-code-assist]

Code Review

This pull request introduces support for comparing arbitrary git references using positional arguments, range syntax (e.g., main..feature), and a new --compare flag across the prep and show commands. The core logic in packages/cli/src/git.ts has been refactored to resolve merge bases and SHAs for these comparisons, supported by new unit tests and updated documentation. Feedback suggests improving the range parsing to allow omitting one side of a range (e.g., ..feature), defaulting to HEAD to better align with standard git behavior.

[cursor]

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