feat: add release-notes command

[marcus]

Summary

• add a developer-facing td release-notes command that drafts changelog-ready markdown from git history
• add repo-scoped git helpers and a dedicated internal/releasenotes package for range resolution, classification, and markdown rendering
• document the new release-note drafting step in the release guide and command docs

Verification

• go test ./internal/git ./internal/releasenotes ./cmd
• go test $(go list ./... | grep -v 'github.com/marcus/td/test/e2e')
• go run . release-notes --from v0.43.0 --to HEAD --version vNEXT | sed -n '1,40p'

Notes

• a raw go test ./... run hangs in github.com/marcus/td/test/e2e in this repo, so the broader verification excludes that package