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