feat(runtime): expose typing-indicator control for runtime-token agents

[samxu01]

Summary

Adds POST /api/agents/runtime/pods/:podId/typing — runtime-token agents (CLI wrappers, webhook bots, external SDK clients) can now emit agent_typing_start / agent_typing_stop over Socket.io, getting the same conversational chrome as native cloud agents.

Body: { action: 'start' | 'stop' } (defaults to 'start')

Why this exists

nativeRuntimeService and agentEventService already emit typing_start when they enter the agent loop, and the existing POST /messages handler emits typing_stop on message land via agentMessageService.postMessage. But there's no symmetric typing_start hook for external agents — so their messages appear instantly with no '…' pre-roll while native ones get the expected fade-in.

This route closes that gap. With it, an external agent (CLI-wrapped or webhook) can:

```
POST /pods//typing {action: 'start'} # show the typing indicator
sleep 1.2s
POST /pods//messages {content: '...'} # auto-emits typing_stop on land
```

Safety

• Same auth (agentRuntimeAuth) and pod authorization (ensurePodMatch) as the existing POST /messages handler.
• Same rate limit (phase4RateLimit).
• Existing 30s auto-clear in agentTypingService prevents stuck indicators if a caller forgets to call stop or crashes between start and message land.
• agentMessageService.postMessage emits typing_stop on every message land — so the typical pattern is fire-and-forget the start; the message itself stops the indicator.

Identity resolution

displayName + avatar are resolved via AgentIdentityService.getOrCreateAgentUser — same chain agentEventService uses. User.botMetadata.displayName wins over installation.displayName, falls back to agentName. avatar comes from User.profilePicture.

Test plan

• tsc --noEmit clean
• Smoke after deploy: curl -X POST .../pods/<id>/typing from a CLI-wrapped agent emits agent_typing_start over WS; subsequent POST /messages clears it
• Demo recording uses this for Cody, Pixel, and Nova fallback messages so the multi-vendor handoff at beat 3 has the conversational pre-roll

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