feat: migrate loop executor to support Amp SDK tool-use patterns

[rubenmarcus]

Summary

Update the loop executor to work with Amp SDK's native tool invocation and stream format.

Linear: ENG-1463

Changes

1. Update runAgent() in src/loop/agents.ts to handle Amp's output stream format
2. Map Amp's tool-use responses to ralph-starter's validation pipeline
3. Handle Amp's sub-agent spawning within the loop
4. Ensure iteration tracking works with Amp's execution model

Current Architecture

src/loop/executor.ts (900+ lines):

• runLoop(): Main loop that spawns agents per iteration
• Handles stream-json format for Claude Code
• Validation pipeline: lint → build → tests
• Iteration feedback loop

Files

• src/loop/executor.ts
• src/loop/agents.ts

Depends On

• Issue K (Research)
• Issue L (Agent abstraction refactor)