ci(release): enforce release-note policy

[lefoulkrod]

What changed

• document the organization release-note and Conventional Commit policy
• require every PR to add a validated visitor-facing fragment or explicitly opt out
• add a tested fragment validator and Keep a Changelog draft generator
• add a dedicated release-note-policy workflow

Why

Site and documentation outcomes should be recorded when their PR is reviewed instead of reconstructed from commit history later.

How it was tested

• node --test tests/release-notes.test.mjs
• node scripts/release-notes.mjs validate-fragments
• ./arbor build
• actionlint on the new workflow

Release note

None: Adds contribution and release-process enforcement; published site content is unchanged.