feat(core): add subagent protocol enhancements

- AgentCapabilitiesSchema with supportsStreaming/Reasoning/Background flags
- supportsStreaming and supportsReasoningStream fields in ProviderSnapshotEntry
- new_task tool: background mode flag for non-blocking subtask dispatch

packages/contracts/src/provider-snapshot.ts

@@ -49,4 +49,6 @@ export interface ProviderSnapshotEntry {
   commands: AgentCommand[];
   error?: string;
   fetchedAt?: string;
+  supportsStreaming?: boolean;
+  supportsReasoningStream?: boolean;
 }