feat: add release-notes command

[marcus]

Summary

• add a built-in td release-notes command that drafts changelog-ready markdown from local git history
• add git helpers and release-note classification/rendering coverage for tag ranges, docs-only commits, and empty output
• fix default --to handling so explicit commit SHAs or branch refs on tagged commits do not walk back past the latest release tag

Testing

• go test ./internal/git ./internal/releasenotes ./cmd
• go test ./...
• go run . release-notes --from v0.43.0 --to HEAD --version vNEXT | head -n 20

Supersedes the earlier release-notes iteration with the explicit --to <sha> tagged-commit regression fixed.

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Automated by nightshift