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/agent-capabilities.ts

@@ -0,0 +1,9 @@
+import { z } from 'zod';
+
+export const AgentCapabilitiesSchema = z.object({
+  supportsStreaming: z.boolean().default(true),
+  supportsReasoningStream: z.boolean().default(false),
+  supportsBackgroundExecution: z.boolean().default(false),
+});
+
+export type AgentCapabilities = z.infer<typeof AgentCapabilitiesSchema>;