feat: add release notes drafter

[marcus]

Scope

• Add td release-notes for deterministic markdown release notes from local git commit ranges.
• Add repo-directory-aware git helpers for ref validation, nearest reachable semver tag discovery, and oldest-first commit listing.
• Add release note grouping/rendering with tests for conventional commits, loose subjects, breaking changes, noisy commit filtering, empty ranges, headings, and section ordering.
• Update README, release guide, and manual website command reference docs.

Assumptions

• "Changes" means local git commits in a selected ref range.
• This does not call GitHub and does not generate AI-authored prose.
• The command resolves git history from the active checkout (cwd, --work-dir, or TD_WORK_DIR) rather than td's shared .todos base directory, so external worktrees draft from their own branch history.

Tests

• go test ./internal/git ./internal/releasenotes ./cmd
• go test ./... initially exposed ambient TD_FEATURE_SYNC_AUTOSYNC=1 / TD_FEATURE_SYNC_CLI=1; reran with those unset.
• env -u TD_FEATURE_SYNC_AUTOSYNC -u TD_FEATURE_SYNC_CLI go test -count=1 ./test/e2e (passes after a transient randomized TestChaosSync seed failure)
• env -u TD_FEATURE_SYNC_AUTOSYNC -u TD_FEATURE_SYNC_CLI go test ./...
• go run . release-notes --from v0.44.0 --to HEAD
• go run . release-notes --from v0.44.0 --to HEAD --version v0.45.0 --date 2026-04-29

Docs

• website/docs/command-reference.md is maintained manually here and was updated for CLI parity.

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