[codex] Calm multi-agent v2 usage prompts (#27037)

## Summary
- tighten the default multi-agent v2 root and subagent usage hints to
bias toward local work
- add a pre-call gate to the v2 spawn_agent description for independent,
bounded, parallelizable subtasks

## Validation
- just fmt
- started just test -p codex-core, but it was interrupted before
completion per follow-up request to commit and push immediately

codex-rs/core/src/tools/handlers/multi_agents_spec.rs

@@ -741,6 +741,7 @@ fn spawn_agent_tool_description_v2(
 You are then able to refer to this agent as `task_3` or `/root/task1/task_3` interchangeably. However an agent `/root/task2/task_3` would only be able to communicate with this agent via its canonical name `/root/task1/task_3`.
 The spawned agent will have the same tools as you and the ability to spawn its own subagents.
 {inherited_model_guidance}
+Only call this tool for a concrete, bounded subtask that can run independently alongside useful local work; otherwise continue locally.
 It will be able to send you and other running agents messages, and its final answer will be provided to you when it finishes.
 The new agent's canonical task name will be provided to it along with the message.
 {concurrency_guidance}"#