Makes release-notes scope mechanical per tier so a release can never
again under-scope its notes and burn the re-mining churn (root cause of
the v3.0.x rewrites).
- **Scope rule + tiers** in the standard (+ codex twin): notes = git
range since the previous release (a major = the whole major), with a
major/minor/hotfix table.
- **Coverage gate** in validate-release-notes.sh: maps the diff to
product areas, FAILS if a heavily-touched area (>=3 files) is
undocumented. Wired into release.yml via --since. It immediately caught
a real gap (v3.0.1 missing the Codex area — fixed here).
- **scaffold-release-notes.sh**: mines the range into a tier-correct
draft so notes start comprehensive.
- 6 new bats (14 total), shellcheck clean, codex parity green. v3.0.1
notes verified to PASS coverage over the real v2.41.1..v3.0.1 diff.
Closes-scenario: ag-nc9#tiers-coverage-gate
Bounded-context: BC5-Runtime
Evidence: scripts/validate-release-notes.sh
Bo