Parallel Agent Architecture

When enabled, the Task tool can now:

1. Spawn multiple agents to work on different aspects of a problem simultaneously
2. Collect results from all agents
3. Use a synthesis agent to combine findings into a cohesive response
4. Display progress for each agent separately in the UI

Example output with parallel agents:

3 total tool uses across 3 agents

Agent 1:
  [tool use progress]

Agent 2:
  [tool use progress]

Synthesis (combining results):
  [synthesis progress]