refactor: replace global channel registry with instance-based Registry and interface-driven adapters

- Replace global channelRegistry singleton with explicit *Registry passed via dependency injection
- Split monolithic manager.go into connection.go (lifecycle), inbound.go (dispatch), outbound.go (pipeline)
- Introduce optional adapter interfaces: ConfigNormalizer, TargetResolver, BindingMatcher
- Move Descriptor() to Adapter interface, remove init()-based registration
- Relocate SessionHub to adapters/local package
- Extract shared UUID/time helpers to internal/db/uuid.go
- Decompose ConfigStore into fine-grained interfaces: ConfigLister, ConfigResolver, BindingStore, SessionStore

docs/swagger.yaml

@@ -178,9 +178,6 @@ definitions:
     properties:
       botID:
         type: string
-      capabilities:
-        additionalProperties: {}
-        type: object
       channelType:
         type: string
       createdAt:
@@ -385,9 +382,6 @@ definitions:
     type: object
   channel.UpsertConfigRequest:
     properties:
-      capabilities:
-        additionalProperties: {}
-        type: object
       credentials:
         additionalProperties: {}
         type: object