chore: gate check (authorized security test)

[Cryptojourney]

Authorized HackerOne research: checking whether fork-PR CI requires maintainer approval. Benign, no payload. — Cryptojourney

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Note

Low Risk
Adds a one-line documentation file with no runtime, security, or CI behavior changes.

Overview
Adds a new GATECHECK.md file with a single line documenting authorized, benign security research to verify whether fork-PR CI requires maintainer approval.

No application code, config, or CI logic is modified—only this marker file for the gate-check test described in the PR description.

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