Effort levels, environment webhooks, and session seeding ship

Several improvements to Claude Managed Agents:

• Set effort level on an agent's model configuration when creating the agent.
• Webhooks now cover environment and memory store lifecycle with four environment.* event types and three memory_store.* event types.
• When creating a session, seed it with initial events by passing initial_events on POST /v1/sessions with up to 50 user.message and user.define_outcome events.
• The version field is now optional when updating an agent. Supply it for optimistic concurrency, or omit it for unconditional updates.
• Session thread event streams now support event deltas via GET /v1/sessions/{session_id}/threads/{thread_id}/stream with the event_deltas[] query parameter.