Add Cursor Agent adapter for headless Phase runs

[maxBRT]

Summary

• Add an agent-agnostic Port (RunPhase) and Cursor headless adapter that spawns a fresh agent subprocess per Phase
• Wire the researched CLI contract (-p --trust --force --approve-mcps --workspace, optional --model, stream-json success subtype, context timeout kill, stderr on failure)
• Cover behavior with a fake executable stub (no live Cursor calls)

Closes #8

Test plan

• go test ./internal/agent/ -v
• go test ./...
• Confirm fake-agent tests cover argv flags, optional model, stderr on non-zero exit, timeout kill, and stream-json success subtype

Made with Cursor