feat: surface sub-agent lifecycle events in chat

[chetan-guevara]

Summary

When Claude uses the Agent tool to spawn a sub-agent, the chat UI now shows real-time activity bubbles tracking the spawn and completion of those agents.

Depends on: #40 (chat tab)

• Spawn acknowledgment — a 🤖 Sub-agent started: [description] bubble appears immediately when an Agent tool call is detected in the stream, so you know a background task was dispatched
• Completion result — when the sub-agent finishes, its result text is posted back to the chat as a new assistant bubble
• Agent activity bubbles are styled distinctly from regular chat messages and are excluded from localStorage persistence while running

Architecture

The stream-json event stream is parsed for two new event types:

• assistant events with content[].type === "tool_use" and name === "Agent" → spawn
• user events with content[].type === "tool_result" matching a tracked tool_use_id → done

These are surfaced as new SSE event types from /api/chat:

• { type: "agent_spawn", id, description }
• { type: "agent_done", id, description, result }

A Map<tool_use_id, description> tracks pending agents across events in streamClaude.

New types

export interface AgentStreamEvent {
  type: "spawn" | "done";
  id: string;
  description: string;
  result?: string;
}

Test plan

• Ask the daemon to do a research task that spawns a sub-agent
• Verify a 🤖 Sub-agent started: ... bubble appears immediately in chat
• Verify the input unblocks so you can continue chatting while the agent runs
• Verify ✅ Sub-agent done: ... bubble appears when complete, followed by the result
• Reload page — verify running agent bubbles are not persisted to localStorage; completed ones are