Closes 2 verification gaps:
1. --sources=github,gitlab flag filters registered sources before sweep
2. Findings persisted to SQLite via storage.SaveFinding after dedup
Also adds Engine.Get() method for source lookup by name.
- Engine.Register/List/SweepAll with ants pool fanout
- ExampleSource emits two deterministic findings (SourceType=recon:example)
- Tests cover Register/List idempotency, SweepAll aggregation, empty-registry,
and Enabled() filtering