Add SQLiteRegistry and RedisRegistry implementations of the AgentRegistry
interface, mirroring the AllowanceStore persistence pattern. SQLiteRegistry
uses modernc.org/sqlite directly with a proper agents table schema, WAL mode,
and UPSERT semantics. RedisRegistry uses go-redis/v9 with TTL-based natural
reaping and SCAN for listing. Add RegistryConfig struct and
NewAgentRegistryFromConfig factory function to config.go.
Full test parity with existing MemoryRegistry tests, including concurrent
access under -race, persistence round-trip, and skip-if-no-Redis pattern.
Co-Authored-By: Virgil <virgil@lethean.io>
