## Summary
- tag legacy multi-agent spawn metrics with `version=v1`
- tag multi-agent v2 spawn metrics with `version=v2`
## Why
`codex.multi_agent.spawn` is emitted by both runtimes, so the existing
metric cannot distinguish v2 adoption from aggregate multi-agent
spawning. The bounded version tag makes that breakdown directly
queryable without changing the counter's success-only semantics.
## Validation
- `just fmt`
- `git diff --check`
- Tests and Clippy were intentionally left to CI.