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/core/src/session/tests.rs

@@ -530,7 +530,9 @@ async fn preview_session_start_hooks(
             transcript_path: None,
             model: "gpt-5.2".to_string(),
             permission_mode: "default".to_string(),
-            source: codex_hooks::SessionStartSource::Startup,
+            target: codex_hooks::StartHookTarget::SessionStart {
+                source: codex_hooks::SessionStartSource::Startup,
+            },
         }),
     )
 }
@@ -1296,7 +1298,9 @@ async fn reload_user_config_layer_refreshes_hooks() -> anyhow::Result<()> {
         transcript_path: None,
         model: "gpt-5.2".to_string(),
         permission_mode: "default".to_string(),
-        source: codex_hooks::SessionStartSource::Startup,
+        target: codex_hooks::StartHookTarget::SessionStart {
+            source: codex_hooks::SessionStartSource::Startup,
+        },
     };
     assert!(session.hooks().preview_session_start(&request).is_empty());
 
@@ -1401,7 +1405,9 @@ async fn refresh_runtime_config_refreshes_hooks() -> anyhow::Result<()> {
         transcript_path: None,
         model: "gpt-5.2".to_string(),
         permission_mode: "default".to_string(),
-        source: codex_hooks::SessionStartSource::Startup,
+        target: codex_hooks::StartHookTarget::SessionStart {
+            source: codex_hooks::SessionStartSource::Startup,
+        },
     };
     assert!(session.hooks().preview_session_start(&request).is_empty());