feat: add changelog command

[marcus]

Summary

• add a read-only td changelog command that drafts paste-ready CHANGELOG.md sections from git history
• add internal/changelog rendering with semver-aware range defaults, commit classification, and meta filtering
• document td changelog as the release drafting workflow and cover the new behavior with CLI and renderer tests

Testing

• go test ./internal/git ./internal/changelog ./cmd
• go test ./...
• go run . changelog --version v0.43.1
• go run . changelog --to v0.43.0 --version v0.43.0

Notes

• codex/release-notes-drafter was not present in this clone or on origin, so this PR targets the equivalent base branch feature/release-notes-drafter-v4 at the same release-notes snapshot.

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Automated by nightshift