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-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -9624,6 +9624,7 @@
           "postCompact",
           "sessionStart",
           "userPromptSubmit",
+          "subagentStart",
           "stop"
         ],
         "type": "string"
@@ -10514,6 +10515,12 @@
             },
             "type": "array"
           },
+          "SubagentStart": {
+            "items": {
+              "$ref": "#/definitions/v2/ConfiguredHookMatcherGroup"
+            },
+            "type": "array"
+          },
           "UserPromptSubmit": {
             "items": {
               "$ref": "#/definitions/v2/ConfiguredHookMatcherGroup"
@@ -10541,6 +10548,7 @@
           "PreToolUse",
           "SessionStart",
           "Stop",
+          "SubagentStart",
           "UserPromptSubmit"
         ],
         "type": "object"