feat: implement Amp SDK multi-agent orchestration for swarm mode

[rubenmarcus]

Summary

Leverage Amp SDK's sub-agent capabilities combined with git worktrees for true parallel multi-agent task execution.

Linear: ENG-1464

Architecture

• Amp SDK supports spawning multiple sub-agents that work on different parts simultaneously
• Combined with git worktrees (Issue A), each sub-agent gets an isolated worktree
• Lead agent coordinates work, assigns subtasks, merges results

Implementation

1. Swarm mode: ralph-starter auto --swarm --agent amp
2. Lead agent decomposes batch into parallelizable subtasks
3. Each subtask spawned as Amp sub-agent in its own worktree
4. Progress display shows all active agents
5. Results merged back to main branch

Files

• src/commands/auto.ts
• src/loop/executor.ts
• src/loop/task-executor.ts

Depends On

• Issue A (Git worktree support)
• Issue L (Agent abstraction refactor)
• Issue M (Executor Amp support)