feat(changelog): U2 — changeset format linter + CI wiring

Add scripts/check-changeset-format.mjs:
- Validates structured changeset schema across .changeset/*.md
- Legacy changesets warn (exit 0) in transition mode
- Structured changesets with missing/invalid fields error (exit 1)
- --strict flag fails on legacy changesets

Wire into package.json (check:changesets, pretest, test:gate) and
pr-checks.yml lint job. Fix parser to not default category on
structured changesets missing the field.

10 linter tests + 18 schema tests all passing.

.github/workflows/pr-checks.yml

@@ -44,6 +44,9 @@ jobs:
       - name: Lint
         run: pnpm lint
 
+      - name: Changeset format
+        run: pnpm check:changesets
+
   typecheck:
     name: Typecheck
     runs-on: ubuntu-latest