Add scripts/lib/distill-release-notes.mjs with:
- distillDeterministic(): groups parsed changesets by category,
renders clean markdown release notes (Keep a Changelog headings)
- buildDistillationPrompt(): builds the context input for AI mode
- DISTILLATION_SYSTEM_PROMPT: system prompt for AI distillation
- Category display order: New → Fixed → Breaking → Security →
Performance → Internal; empty categories omitted
The AI mode (createFnAgent integration) is deferred to U4 where the
engine runtime is available. The deterministic mode is the fallback
contract: it always produces valid output regardless of model access.
14 tests covering category grouping, ordering, empty/edge cases,
prompt building, and legacy entry handling.
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