perf: hook subprocess fork per tool call

[csmith49]

Problem

Each hook execution spawns a new subprocess via subprocess.Popen (async) or subprocess.run (sync). For a conversation with 3 registered hooks and 50 tool calls, that's 150 process forks. Each fork includes full JSON serialization of the event as stdin.

Location: hooks/executor.py:175, 214

Evidence

Code review of hooks/executor.py. The AsyncProcessManager.cleanup_expired() does a linear scan of all tracked processes before every hook execution.

Severity: High

Impact of fix

Eliminates hundreds of fork+exec cycles per conversation.

Suggested fixes

• Long-running hook process with stdin/stdout IPC (send events as JSON lines, receive responses).
• For trusted hooks: in-process execution via importable Python functions.

Related issues and PRs

• [Feature]: Agent-based hook evaluation (HookType.AGENT) #2864 — Agent-based hook evaluation (HookType.AGENT) (open issue, alternative in-process hook model)
• feat(hooks): add HookType.AGENT for agent-based hook evaluation #3052 — feat(hooks): add HookType.AGENT for agent-based hook evaluation (open PR, implementation of in-process hooks)

Discovered during profiling investigation (code review), May 2026. openhands-sdk v1.19.1.