Adds scripts/build_decisions_catalog.py, which renders docs/decisions/README.md
as a compact table of only 'active' decision records (sorted by key), and its
test scripts/tests/test_build_catalog.py. Supports --check for CI drift
detection. No decision records are migrated yet, so the generated catalog is
currently empty (banner + header only) — expected at this stage.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>