fix: exit process when ACP connection closes

[WayChan]

Summary

• Return AgentSideConnection from runAcp() instead of discarding it
• Listen for connection.closed and call process.exit(0) on resolution
• Fixes orphan process accumulation in oneshot mode (e.g. via ACPX)

Problem

When used in oneshot mode, claude-agent-acp does not exit after the client closes the stdin pipe. process.stdin.resume() keeps the Node.js event loop alive indefinitely. Over time, orphan processes accumulate on the host (~97MB RSS each), eventually exhausting system resources.

Root cause

runAcp() creates an AgentSideConnection but discards the reference. The Connection.#receive() loop detects stdin EOF and fires abortController.abort(), resolving connection.closed — but no code listens for it to trigger process.exit().

Fix

1. acp-agent.ts: runAcp() now returns the AgentSideConnection instance
2. index.ts: Awaits connection.closed and calls process.exit(0)

Impact

• Oneshot sessions: Process now exits cleanly when the client closes stdin (EOF)
• Persistent sessions: Unaffected — the client keeps stdin open, so connection.closed does not fire until explicit disconnect

Test plan

• Verified oneshot: process exits within 1s after stdin EOF
• Verified persistent: process remains alive while stdin is open
• Measured idle RSS: ~97MB → freed on exit
• Tested with ACPX runtime (OpenClaw gateway) in production

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