chore: prepare v2.3.1 release

[helizaga]

Summary

• Bump version to 2.3.1
• Add v2.3.1 CHANGELOG entry (1 new adapter, 3 fixes, 1 performance improvement)
• Backfill missing v2.3.0 CHANGELOG entry
• Fix comparison links at bottom of CHANGELOG

Changes in v2.3.1

• Added: Google Antigravity adapter (Add Google's Antigravity support #121)
• Changed: CoW cloning for directory copies (perf: use CoW (copy-on-write) cloning for directory copies #122)
• Fixed: gtr/coreutils naming conflict + cd completions (fix: resolve gtr/coreutils naming conflict and add cd completions #125), main repo root resolution from worktrees (fix: resolve main repo root correctly from inside worktrees #126), resolve_target fallback for external worktrees (fix: resolve_target fallback via git worktree list for external worktrees #128)

After merge

git tag -a v2.3.1 -m "v2.3.1"
git push origin v2.3.1
gh release create v2.3.1 --title "v2.3.1" --generate-notes --notes-start-tag v2.3.0

Summary by CodeRabbit

• New Features

  • Added Google Antigravity adapter support

• Changed

  • Implemented copy-on-write cloning for directory copies on supported filesystems

• Bug Fixes

  • Fixed naming conflicts and improved cd completions
  • Fixed repository root resolution from inside worktrees
  • Fixed external worktree fallback resolution

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

This PR releases version 2.3.1, updating the version constant and changelog. The release introduces Google Antigravity adapter support, copy-on-write cloning for directory operations, and fixes for naming conflicts, completions, repository resolution, and worktree handling.

Changes

Cohort / File(s)	Summary
Release Version Bump bin/git-gtr	Increments GTR_VERSION constant from "2.3.0" to "2.3.1".
Changelog Update CHANGELOG.md	Adds [2.3.1] release section with new Google Antigravity adapter, copy-on-write cloning for directories, and fixes for gtr/coreutils naming conflicts, cd completions, repo root resolution, and worktree handling. Updates release link references.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A version bump hops into place,
Changelog updates at record pace,
New adapters float without a trace,
Bugs fixed with gravity-defying grace! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately summarizes the main purpose of the pull request: preparing a v2.3.1 release by updating version numbers and changelog entries.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch release/v2.3.1

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