Add Codex usage guidelines to CLAUDE.md

[jdalton]

Summary

• Add Codex usage section to CLAUDE.md establishing conventions for how Codex should be used in this repo
• Consistent with conventions in socket-btm and ultrathink repos

Details

Codex is for advice and critical assessment only — never for making code changes directly. This prevents incidents like the inline asm prefetch regression seen in ultrathink.

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Note

Low Risk
Low risk documentation-only change that updates contributor/assistant workflow guidance without affecting runtime code or behavior.

Overview
Adds a new Codex Usage section to CLAUDE.md that explicitly restricts Codex to advice/review/analysis and forbids using it to directly implement or modify code, plus guidance on when to consult it proactively.

No functional code changes; this PR only updates project instructions.

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