feat: add Kiro CLI as built-in agent with proper process group cleanup

[thepagent]

Summary

Adds Kiro CLI as a built-in agent and fixes orphan child processes caused by wrapper binary patterns.

Problem

kiro-cli acp is a wrapper that forks kiro-cli-chat as the actual ACP server. The old agent.kill() only killed the wrapper — the child became an orphan and accumulated over time.

spawn "kiro-cli acp"  (pid N)
        └─ forks "kiro-cli-chat acp"  (pid N+7)  ← actual ACP server

agent.kill()  →  kills pid N only
                 pid N+7 reparented to init  💀 orphan

This affects any agent using a wrapper binary pattern.

Fix

Spawn with detached: true so the agent becomes a process group leader. On close, use process.kill(-pgid, SIGTERM) to kill the entire group.

process.kill(-N, SIGTERM)  →  kills pgid N (includes N+7)  ✅

Falls back to child.kill() if process group kill is unsupported.

Tests

Two new tests in test/process-group-kill.test.ts prove the fix:

1. process group kill terminates parent and grandchild (no orphan) — verifies the fix works
2. killing only parent pid leaves grandchild as orphan (demonstrates the bug) — reproduces the original bug

Changes

• src/agent-registry.ts — add kiro: "kiro-cli acp" as built-in agent
• src/client.ts — detached: true on spawn + process.kill(-pgid) in terminateAgentProcess
• src/session-runtime.ts — add onAgentPid callback to runQueuedTask / runSessionPrompt
• test/agent-registry.test.ts — update for 6 agents
• test/process-group-kill.test.ts — new: prove process group kill vs orphan

Related

• Builds on feat: add Kiro CLI as built-in agent #40 (kiro agent registry)

[osolmaz]

Is this ready and tested? happy to merge it