Tighten hook output event schemas

[abhinav-oai]

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