[codex] Restore v1 delegation guidance

[aibrahim-oai]

Summary

• restore the v1 clarification that requests for depth, research, or investigation do not authorize subagent spawning
• restore guidance for keeping critical-path, urgent, tightly coupled, or difficult work local
• update the focused v1 tool-search and spawn-description coverage

Why

PR #27919 simplified the v1 spawn_agent prompt by removing its delegation decision guidance. That left the authorization rule intact, but removed the instructions that constrained what should be delegated after spawning was authorized.

Restore those guardrails while preserving later support for explicit delegation authorization from applicable AGENTS.md and skill instructions. Multi-agent v2 prompts are unchanged.

User impact

Models using the v1 multi-agent tool surface receive clearer guidance to delegate independent side work while keeping blocking work on the main rollout.

Validation

• just fmt
• git diff --check
• tests not run locally per repository guidance; CI will validate the focused coverage