docs: simplify CLAUDE shim

[jbeckwith-oai]

Summary

• normalize CLAUDE.md to the single-line @AGENTS.md shim

Test Plan

• git diff --check

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👍 All Clear

Reviewed a documentation-only change redirecting CLAUDE.md to AGENTS.md. There are no source code modifications, no LLM calls, and no changes to agent capabilities or data flows. Accordingly, there are no LLM security implications introduced by this PR.

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