#1537 shipped with a manual test checklist only. Add automated tests that
corrupt a graph.json and assert the actionable RuntimeError at all three load
paths (build_merge, affected.load_graph, diagnostics._read_json_file) plus a
happy-path guard. Also record the six merged small fixes in the changelog.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
safishamsi