Adds scripts/compliance/emit-retention-policy.mjs which walks Payload
collection files, validates purgeSchedule is declared on every
collection, and emits deterministic YAML to
compliance/retention-policy.yml. Supports --print and --check modes.
Wires compliance:retention-policy root package script. Ships 19 unit
tests covering validation, builder, YAML rendering, and diff modes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>