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/hooks/src/lib.rs

@@ -15,7 +15,7 @@ pub use declarations::PluginHookDeclaration;
 pub use declarations::plugin_hook_declarations;
 pub use engine::HookListEntry;
 /// Hook event names as they appear in hooks JSON and config files.
-pub const HOOK_EVENT_NAMES: [&str; 8] = [
+pub const HOOK_EVENT_NAMES: [&str; 9] = [
     "PreToolUse",
     "PermissionRequest",
     "PostToolUse",
@@ -23,6 +23,7 @@ pub const HOOK_EVENT_NAMES: [&str; 8] = [
     "PostCompact",
     "SessionStart",
     "UserPromptSubmit",
+    "SubagentStart",
     "Stop",
 ];
 
@@ -31,13 +32,14 @@ pub const HOOK_EVENT_NAMES: [&str; 8] = [
 /// Other events can appear in hooks JSON, but Codex ignores their matcher
 /// fields because those events do not dispatch against a tool, compaction
 /// trigger, or session-start source.
-pub const HOOK_EVENT_NAMES_WITH_MATCHERS: [&str; 6] = [
+pub const HOOK_EVENT_NAMES_WITH_MATCHERS: [&str; 7] = [
     "PreToolUse",
     "PermissionRequest",
     "PostToolUse",
     "PreCompact",
     "PostCompact",
     "SessionStart",
+    "SubagentStart",
 ];
 
 pub use events::compact::PostCompactRequest;
@@ -54,6 +56,7 @@ pub use events::pre_tool_use::PreToolUseRequest;
 pub use events::session_start::SessionStartOutcome;
 pub use events::session_start::SessionStartRequest;
 pub use events::session_start::SessionStartSource;
+pub use events::session_start::StartHookTarget;
 pub use events::stop::StopOutcome;
 pub use events::stop::StopRequest;
 pub use events::user_prompt_submit::UserPromptSubmitOutcome;
@@ -83,6 +86,7 @@ pub fn hook_event_key_label(event_name: HookEventName) -> &'static str {
         HookEventName::PostCompact => "post_compact",
         HookEventName::SessionStart => "session_start",
         HookEventName::UserPromptSubmit => "user_prompt_submit",
+        HookEventName::SubagentStart => "subagent_start",
         HookEventName::Stop => "stop",
     }
 }