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/schema/generated/subagent-start.command.input.schema.json

@@ -0,0 +1,63 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "definitions": {
+    "NullableString": {
+      "type": [
+        "string",
+        "null"
+      ]
+    }
+  },
+  "properties": {
+    "agent_id": {
+      "type": "string"
+    },
+    "agent_type": {
+      "type": "string"
+    },
+    "cwd": {
+      "type": "string"
+    },
+    "hook_event_name": {
+      "const": "SubagentStart",
+      "type": "string"
+    },
+    "model": {
+      "type": "string"
+    },
+    "permission_mode": {
+      "enum": [
+        "default",
+        "acceptEdits",
+        "plan",
+        "dontAsk",
+        "bypassPermissions"
+      ],
+      "type": "string"
+    },
+    "session_id": {
+      "type": "string"
+    },
+    "transcript_path": {
+      "$ref": "#/definitions/NullableString"
+    },
+    "turn_id": {
+      "description": "Codex extension: expose the active turn id to internal turn-scoped hooks.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "agent_id",
+    "agent_type",
+    "cwd",
+    "hook_event_name",
+    "model",
+    "permission_mode",
+    "session_id",
+    "transcript_path",
+    "turn_id"
+  ],
+  "title": "subagent-start.command.input",
+  "type": "object"
+}