Add release-notes command

[marcus]

Summary

• add the td release-notes command with git-backed markdown drafting and release docs updates
• include git helpers and tests for semver tag discovery, commit listing, and release-note rendering
• fix retagged release handling so same-commit semver tags draft an empty range instead of duplicating already-released changes

Testing

• go test ./internal/git ./internal/releasenotes ./cmd
• go test ./...
• go run . release-notes --from v0.43.0 --to HEAD --version vNEXT
• go run . release-notes --work-dir --to v0.2.1 --version v0.2.1