Move the Finding type that was duplicated between patrol and investigation
packages into a new internal/ai/finding package. Both packages now use
type aliases for backwards compatibility.
Changes:
- Add internal/ai/finding/finding.go as the canonical Finding definition
- Update investigation.Finding to be a type alias for finding.Finding
- Update InvestigationFinding in patrol.go to be a type alias
- Add ToCoreFinding() method to ai.Finding for type conversion
- Simplify investigation_adapter.go (no more field-by-field copies)
- Add e2e integration test for patrol investigation flow
