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/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__hooks_popup_shows_list_diagnostics.snap

@@ -18,6 +18,7 @@ expression: popup
   SessionStart          0           0           When a new session starts
   UserPromptSubmit      0           0           When the user submits a prompt
   SubagentStart         0           0           When a subagent is created
+  SubagentStop          0           0           Right before a subagent ends its turn
   Stop                  0           0           Right before Codex ends its turn
 
   Press enter to view hooks; esc to close

codex-rs/tui/src/chatwidget/tests/helpers.rs

@@ -1572,6 +1572,7 @@ fn hook_event_label(event_name: codex_app_server_protocol::HookEventName) -> &'s
         codex_app_server_protocol::HookEventName::SessionStart => "SessionStart",
         codex_app_server_protocol::HookEventName::UserPromptSubmit => "UserPromptSubmit",
         codex_app_server_protocol::HookEventName::SubagentStart => "SubagentStart",
+        codex_app_server_protocol::HookEventName::SubagentStop => "SubagentStop",
         codex_app_server_protocol::HookEventName::Stop => "Stop",
     }
 }