#11951 Make OpenCode cmux hooks nonblocking

Summary

• Dispatch generated OpenCode lifecycle hooks through a bounded asynchronous queue.
• Limit hook fan-out to four active children and 256 pending entries, coalescing repeated same-session transitions.
• Deduplicate session-start delivery from repeated session.updated events.
• Release queue lanes immediately when a timed-out hook is killed.

Verification

• swiftc -frontend -parse CLI/cmux.swift
• swiftc -frontend -parse cmuxTests/OpenCodeHookRegressionTests.swift
• Generated plugin node --check --input-type=module
• Runtime fixture: callback returned in about 2 ms with a 1 second fake cmux process.
• Runtime overload fixture: 130 accepted starts and 130 matching ends from 301 sessions.
• Tagged fleet build attempted with ocnb2; blocked by existing Sources/Cloud/VMTunnelManager.swift:329 error (VMTunnelManager has no member completedConfig).

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Summary by cubic

Makes OpenCode lifecycle hooks nonblocking so they no longer block OpenCode's event loop.

• Replaces synchronous spawnSync hook calls with a bounded async queue (4 active children, 256 pending entries).
• Coalesces repeated same-session lifecycle transitions and dedupes session-start delivery from session.created and session.updated.
• Drops hooks when the queue is full, except session-end for sessions with a previously accepted start.
• Releases queue lanes immediately when a hook times out after 5 seconds.

Summary by CodeRabbit

• Performance

  • OpenCode session hook delivery is now asynchronous, preventing session events from blocking while external processes respond.
  • Hook processing remains ordered per session while supporting concurrent activity across sessions.

• Reliability

  • Repeated session updates no longer trigger duplicate session-start events.
  • Queue limits, timeouts, and event coalescing improve behavior during high activity.
  • Accepted session lifecycle events are reliably paired and delivered.