docs(agents): adopt the test authoring gate

[code-yeongyu]

Summary

Adopts the test authoring gate from the test audit in a new root AGENTS.md, together with the facts agents need to run and add tests here. These were previously only in the user-facing README:

• the Bun and Node runners run the same explicit file lists, so a new test file must be added to both lists;
• the cloak-profile script;
• the headless-shell environment variables;
• the hygiene-scan rule.

There were no earlier agent test rules, so nothing needed reconciling.

What the gate says

The four authoring questions, the junk-pattern list and the retention bar, copied word for word from the audit's shared gate text. All eight audited repositories get the same text, with only the heading level and runner names adapted.

Validation

• bun test test/hygiene.test.mjs and node --test test/hygiene.test.mjs both pass with the new file, so it contains no absolute home path or private host.
• Documentation only: no code, test or workflow change.

Refs #28

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Summary by cubic

Adds a root AGENTS.md with the test authoring gate and the runner and hygiene notes agents need to run and add tests here, previously only in the user-facing README.

• Covers the explicit test file lists in both package.json scripts, the cloak-profile script, headless-shell environment variables, and the hygiene-scan rule.
• Includes the full gate text (four questions, junk patterns, retention bar) copied from the audit; there were no earlier agent test rules to reconcile.
• Documentation only; no code, test, or workflow changes, and the new file passes the hygiene scan on both runners.

Refs #28

^{Written for commit bc0d654. Summary will update on new commits.}