feat: add release-notes command

[marcus]

Summary

• add a local-first \ command that drafts markdown notes from git history
• extend \ with latest-tag/range resolution, structured release-note commit data, and focused temp-repo tests
• document the new release workflow in the release guide, README, and website command reference

Testing

• go test ./internal/git ./cmd
• go run . release-notes --version vNEXT --date 2026-04-02

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