Remove the fail_after note from AGENTS.md

[Kludex]

Drops the AGENTS.md rule that told agents to wrap every indefinite wait in anyio.fail_after(5). In in-memory tests it mostly adds nesting without catching anything.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-17T13:47:33.577901Z	15bc141	PR opened

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[cubic-dev-ai]

No issues found across 1 file

_{Turn on auto-fix | Re-trigger cubic}

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[Kludex]

Closing: the rule only targets event.wait() / stream.receive(), which is reasonable. It was misapplied in #3726, not wrong.