Add `onAgentActivityChanged` SDK hook

[willwashburn]

Add a first-class SDK hook named onAgentActivityChanged so app clients do not need to infer “thinking” from raw broker events themselves.

Goal:

• Add onAgentActivityChanged to the high-level AgentRelay API in the SDK.
• This should be an SDK-level derived signal built from existing broker events.
• Prefer not to add any new broker protocol events unless you conclude the existing events are insufficient. If you do change protocol, explain why.

Why:

• Today clients have to combine delivery_*, relay_inbound, and agent_idle to show a simple “agent is active/thinking” badge.
• That logic belongs in the SDK, not in each app.

Relevant files:

• packages/sdk/src/relay.ts
• packages/sdk/src/protocol.ts
• src/pty_worker.rs

Current event context:

• AgentRelay already has hooks like onMessageReceived, onMessageSent, onDeliveryUpdate, onAgentIdle, and onWorkerOutput.
• Broker events already include:
  • delivery_queued
  • delivery_injected
  • delivery_active
  • delivery_ack
  • delivery_failed
  • relay_inbound
  • agent_idle
  • agent_exited
  • agent_released

Requested API:

• Add a new hook on AgentRelay, something like:

  interface AgentActivityChange {
  name: string;
  active: boolean;
  pendingDeliveries: number;
  reason:
  | 'delivery_queued'
  | 'delivery_injected'
  | 'delivery_active'
  | 'delivery_ack'
  | 'delivery_failed'
  | 'relay_inbound'
  | 'agent_idle'
  | 'agent_exited'
  | 'agent_released';
  eventId?: string;
  }

  onAgentActivityChanged: EventHook = null;

Semantics:

• This is an “activity” signal, not a literal cognition signal.
• active=true should mean: the SDK has seen an in-flight delivery for the agent that has not yet been closed out.
• Mark an agent active when a delivery enters activity:
  • delivery_queued
  • delivery_injected
  • delivery_active
  • delivery_ack
• Mark an agent inactive when activity is closed out:
  • relay_inbound from that agent
  • agent_idle
  • delivery_failed when no pending deliveries remain
  • agent_exited
  • agent_released
• Maintain per-agent pending delivery state so concurrent deliveries behave correctly.
• Do not emit duplicate callbacks when the derived active state has not changed.

Notes:

• delivery_ack is not proof the model is actively reasoning; it is still useful as part of the broader “activity” signal.
• If you think a different state shape is better, keep the API simple and app-facing.

Tests:

• Add focused SDK tests covering:
  • emits active on first delivery event
  • does not emit repeated active transitions while already active
  • emits inactive on relay_inbound
  • emits inactive on agent_idle
  • concurrent deliveries do not flip inactive too early
  • emits inactive on exit/release
  • delivery_failed only clears activity when the last pending delivery is gone

Docs:

• Add a short SDK example showing how a UI could render a “thinking” badge from onAgentActivityChanged.

Output:

• Implement the change
• Run relevant tests
• Summarize semantics/tradeoffs
• List changed files

This keeps the change SDK-side and avoids pushing relay/broker event inference into each app.