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
Documentation-only addition with no runtime, CI config, or security-sensitive code changes.

Overview
Adds a new GATECHECK.md file with a single line stating this is authorized, benign security research to verify whether fork-PR CI runs without maintainer approval.

There are no application, workflow, or dependency changes—only this marker file for the gate-check test described in the PR description.

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