Add SubagentStop hook

[abhinav-oai]

What

SubagentStop runs when a thread-spawned subagent turn is about to finish. Thread-spawned subagents use SubagentStop instead of the normal root-agent Stop hook.

Configured handlers match on agent_type. Hook input includes the normal stop fields plus:

• agent_id: the child thread id.
• agent_type: the resolved subagent type.
• agent_transcript_path: the child subagent transcript path.
• transcript_path: the parent thread transcript path.
• last_assistant_message: the final assistant message from the child turn, when available.
• stop_hook_active: true when the child is already continuing because an earlier stop-like hook blocked completion.

SubagentStop shares the same completion-control semantics as Stop, scoped to the child turn:

• No decision allows the child turn to finish.
• decision: "block" with a non-empty reason records that reason as hook feedback and continues the child with that prompt.
• continue: false stops the child turn. If stopReason is present, Codex surfaces it as the stop reason.

Lifecycle Scope

Only thread-spawned subagents run SubagentStop.

Internal/system subagents such as Review, Compact, MemoryConsolidation, and Other do not run normal Stop hooks and do not run SubagentStop. This avoids exposing synthetic matcher labels for internal implementation paths.

Stack

1. Add SubagentStart hook #22782: add SubagentStart.
2. This PR: add SubagentStop.
3. [codex] Add subagent identity to hook inputs #22882: add subagent identity to normal hook inputs.

[abhinav-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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