Tighten hook output event schemas (#24962)

# Why

Fixes #23993.

Hook command output schemas are published as the contract for hook
authors and schema-driven tooling. The event-specific output schemas
previously described `hookSpecificOutput.hookEventName` as the global
`HookEventNameWire` enum, so a `pre-tool-use.command.output` schema
would validate mismatched values like `PostToolUse`. That made the
schemas less precise than the intended event-specific contract.

# What

Constrain each hook-specific output schema to the matching literal
`hookEventName` value, mirroring the existing input-schema shape.

Also split `SubagentStartHookSpecificOutputWire` from the session-start
output wire so `subagent-start.command.output.schema.json` can emit
`const: "SubagentStart"` instead of sharing the session-start
definition.

# Verification

- `cargo nextest run -p codex-hooks`
- `just fix -p codex-hooks`
- `just argument-comment-lint -p codex-hooks -- --all-targets`

codex-rs/hooks/schema/generated/session-start.command.output.schema.json

@@ -2,21 +2,6 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "additionalProperties": false,
   "definitions": {
-    "HookEventNameWire": {
-      "enum": [
-        "PreToolUse",
-        "PermissionRequest",
-        "PostToolUse",
-        "PreCompact",
-        "PostCompact",
-        "SessionStart",
-        "UserPromptSubmit",
-        "SubagentStart",
-        "SubagentStop",
-        "Stop"
-      ],
-      "type": "string"
-    },
     "SessionStartHookSpecificOutputWire": {
       "additionalProperties": false,
       "properties": {
@@ -25,7 +10,8 @@
           "type": "string"
         },
         "hookEventName": {
-          "$ref": "#/definitions/HookEventNameWire"
+          "const": "SessionStart",
+          "type": "string"
         }
       },
       "required": [