runtime: detached warm session owners and opencode ACP mode

[dutifulbob]

Summary

This PR moves acpx to detached warm session owners so prompt calls no longer block on queue-owner idle TTL.

It also fixes OpenCode invocation to ACP mode.

What changed

• Run OpenCode in ACP mode from registry defaults:

  • opencode now maps to npx -y opencode-ai acp

• Detached queue-owner architecture:

  • caller path no longer runs the owner loop
  • caller submits to an active owner or spawns detached __queue-owner
  • prompt returns when the turn completes (no hidden idle wait)

• Internal owner runtime wiring:

  • hidden internal owner command path in CLI
  • owner process reuses existing queue protocol and turn controller

• Queue lease hardening:

  • lock state now includes ownerGeneration, heartbeatAt, and queueDepth
  • owner heartbeat refresh while alive
  • stale owner heartbeat detection and reclaim

• Status observability:

  • status now reports owner health fields:
    • ownerStatus
    • ownerPid
    • ownerGeneration
    • ownerHeartbeatAt
    • ownerQueueDepth

• Test updates:

  • integration test: prompt exits while owner remains warm and reusable
  • queue-ipc test: stale heartbeat lock reclamation
  • harness updates for detached-owner timing behavior

Validation

• npm run format:check
• npm run lint
• npm test
• manual smoke with mock agent:
  • first prompt returns immediately
  • owner stays alive after prompt
  • second prompt reuses warm owner

Breaking behavior

This is intentionally a forward-only alpha refactor for owner runtime/lock behavior.
No backward-compatibility shims were added for legacy owner lock shape.