Add SubagentStop hook (#22873)

# What

<img width="1792" height="1024" alt="image"
src="https://github.com/user-attachments/assets/8f81d232-5813-4994-a61d-e42a05a93a3e"
/>

`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. #22782: add `SubagentStart`.
2. This PR: add `SubagentStop`.
3. #22882: add subagent identity to normal hook inputs.

codex-rs/config/src/hook_config.rs

@@ -47,6 +47,8 @@ pub struct HookEventsToml {
     pub user_prompt_submit: Vec<MatcherGroup>,
     #[serde(rename = "SubagentStart", default)]
     pub subagent_start: Vec<MatcherGroup>,
+    #[serde(rename = "SubagentStop", default)]
+    pub subagent_stop: Vec<MatcherGroup>,
     #[serde(rename = "Stop", default)]
     pub stop: Vec<MatcherGroup>,
 }
@@ -62,6 +64,7 @@ impl HookEventsToml {
             session_start,
             user_prompt_submit,
             subagent_start,
+            subagent_stop,
             stop,
         } = self;
         pre_tool_use.is_empty()
@@ -72,6 +75,7 @@ impl HookEventsToml {
             && session_start.is_empty()
             && user_prompt_submit.is_empty()
             && subagent_start.is_empty()
+            && subagent_stop.is_empty()
             && stop.is_empty()
     }
 
@@ -85,6 +89,7 @@ impl HookEventsToml {
             session_start,
             user_prompt_submit,
             subagent_start,
+            subagent_stop,
             stop,
         } = self;
         [
@@ -96,6 +101,7 @@ impl HookEventsToml {
             session_start,
             user_prompt_submit,
             subagent_start,
+            subagent_stop,
             stop,
         ]
         .into_iter()
@@ -104,7 +110,7 @@ impl HookEventsToml {
         .sum()
     }
 
-    pub fn into_matcher_groups(self) -> [(HookEventName, Vec<MatcherGroup>); 9] {
+    pub fn into_matcher_groups(self) -> [(HookEventName, Vec<MatcherGroup>); 10] {
         [
             (HookEventName::PreToolUse, self.pre_tool_use),
             (HookEventName::PermissionRequest, self.permission_request),
@@ -114,6 +120,7 @@ impl HookEventsToml {
             (HookEventName::SessionStart, self.session_start),
             (HookEventName::UserPromptSubmit, self.user_prompt_submit),
             (HookEventName::SubagentStart, self.subagent_start),
+            (HookEventName::SubagentStop, self.subagent_stop),
             (HookEventName::Stop, self.stop),
         ]
     }