feat: add release notes drafter

[marcus]

Scope

• Add td release-notes to draft deterministic markdown from local git commits.
• Add repo-dir-aware git helpers for ref resolution, nearest reachable semver tag selection, and oldest-first commit listing.
• Add release-note grouping/rendering for breaking changes, features, bug fixes, documentation, and improvements.
• Update README, release guide, and manual website command reference docs.

Assumptions

• "Changes" means commits in a local git ref range.
• This first slice does not call GitHub APIs and does not generate AI-authored prose.

Tests

• go test ./internal/git ./internal/releasenotes ./cmd
• go test ./... failed with ambient TD_FEATURE_SYNC_AUTOSYNC=1 and TD_FEATURE_SYNC_CLI=1 because internal/features expects default feature state.
• env -u TD_FEATURE_SYNC_AUTOSYNC -u TD_FEATURE_SYNC_CLI go test ./...
• TD_ANALYTICS=false go run . release-notes --from v0.44.0 --to HEAD
• TD_ANALYTICS=false go run . release-notes --from v0.44.0 --to HEAD --version v0.45.0 --date 2026-04-29

Docs

• website/docs/command-reference.md appears manually maintained, so it was refreshed in this PR.

[marcus]

Closing as a duplicate. An automated agent opened 18 separate PRs for the same changelog / release-notes drafting command between March and May 2026. Keeping #176 as the single representative; closing the rest unreviewed.