Add promise default fallback rule

[benvinegar]

Summary

• add the new defensive.promise-default-fallbacks rule for promise .catch() handlers that suppress rejections with cheap sentinel defaults or empty handlers
• register the rule in the default registry and document it in a new rule README plus the main README rule list
• add focused tests for the new rule and keep the existing heuristic/fixture regression coverage green
• include the full pinned benchmark artifacts and autoresearch notes showing the rule's strong full-set benchmark signal

Benchmark signal

• full pinned benchmark signal score: 0.9773662551440329
• best separating metric: findings / file (0.9876543209876543)
• hit rate: 9/9 AI repos vs 4/9 mature OSS repos
• full report: reports/autoresearch-candidate-rule.md

Validation

• bun test tests/heuristics.test.ts tests/promise-default-fallbacks.test.ts tests/rule-signal-benchmark.test.ts
• bun test tests/fixtures-regression.test.ts tests/benchmark.test.ts
• bun run lint

This PR description was generated by Pi using GPT-5

[benvinegar]

Closing as superseded.

The useful parts of this PR have already landed on main through later merged work, including the per-rule benchmark infrastructure and the defensive.promise-default-fallbacks rule itself. After rebasing this branch onto current main, it is now effectively empty relative to main, so there is no remaining merge delta here.

This comment was generated by Pi using GPT-5 Codex