feat: add td release-notes command

[marcus]

Summary

• add a built-in td release-notes command that drafts CHANGELOG-ready markdown from local git history
• extend git helpers for repo resolution, semver tag selection, ref validation, and chronological non-merge commit listing with touched files
• document the new release-notes flow in the README, release guide, and command reference

Testing

• go test ./internal/git ./internal/releasenotes ./cmd
• go run . release-notes --from v0.43.0 --to HEAD --version vNEXT