Add SubagentStart hook (#22782)

# What

`SubagentStart` runs once when Codex creates a thread-spawned subagent,
before that child sends its first model request. Thread-spawned
subagents use `SubagentStart` instead of the normal root-agent
`SessionStart` hook.

Configured handlers match on the subagent `agent_type`, using the same
value passed to `spawn_agent`. When no agent type is specified, Codex
uses the default agent type.

Hook input includes the normal session-start fields plus:

- `agent_id`: the child thread id.
- `agent_type`: the resolved subagent type.

`SubagentStart` may return `hookSpecificOutput.additionalContext`. That
context is added to the child conversation before the first model
request.

# Lifecycle Scope

Only thread-spawned subagents run `SubagentStart`.

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

Also the `SessionStart` hook no longer fires for subagents, this matches
behavior with other coding agents' implementation

# Stack

1. This PR: add `SubagentStart`.
2. #22873: add `SubagentStop`.
3. #22882: add subagent identity to normal hook inputs.

codex-rs/app-server/src/request_processors/config_processor.rs

@@ -454,6 +454,7 @@ fn map_hooks_requirements_to_api(hooks: ManagedHooksRequirementsToml) -> Managed
         post_compact,
         session_start,
         user_prompt_submit,
+        subagent_start,
         stop,
     } = hooks;
 
@@ -467,6 +468,7 @@ fn map_hooks_requirements_to_api(hooks: ManagedHooksRequirementsToml) -> Managed
         post_compact: map_hook_matcher_groups_to_api(post_compact),
         session_start: map_hook_matcher_groups_to_api(session_start),
         user_prompt_submit: map_hook_matcher_groups_to_api(user_prompt_submit),
+        subagent_start: map_hook_matcher_groups_to_api(subagent_start),
         stop: map_hook_matcher_groups_to_api(stop),
     }
 }