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

@@ -17,6 +17,7 @@ expression: popup
   PostCompact           0           0           After context compaction
   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
   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

@@ -1571,6 +1571,7 @@ fn hook_event_label(event_name: codex_app_server_protocol::HookEventName) -> &'s
         codex_app_server_protocol::HookEventName::PostCompact => "PostCompact",
         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::Stop => "Stop",
     }
 }