Build context bundles for shadow signal reviews

[Pigbibi]

Summary

• add a dependency-free context bundle builder for long-horizon shadow reviews
• embed the generated context bundle in the dated shadow-signal issue before dispatching CodexAuditBridge
• keep scheduled runs observable even when the external price download fails by writing a degraded error context bundle

Validation

• python -m pytest -q
• python scripts/validate_latest_signal.py examples/latest_signal.example.json
• python scripts/build_context_bundle.py --prices examples/price_history.example.csv --symbols QQQ --output /tmp/context_bundle.json
• python scripts/build_context_bundle.py --symbols QQQ --lookback-days 420 --output /tmp/context_bundle_live_or_error.json --allow-download-errors