[codex] Document local test fixture secrets

[albert-einshutoin]

Summary

Documents the fixture secrets required for local generated-artifact and release-gate test workflows.

Why

The local README test commands previously omitted the environment variables that CI sets. EDGE_ADMIN_TOKEN is needed for generated artifacts with the built-in admin static_token gate, and ORIGIN_SECRET is needed by origin-auth fixture policies in broader drift/release checks.

Changes

• Adds fixture secret exports before the README local test commands in English and Japanese.
• Expands the test strategy docs with the same local workflow prerequisite.
• Clarifies that these values are only for local/CI validation and production builds must use deployment secrets.

Validation

• EDGE_ADMIN_TOKEN=ci-build-token-not-for-deploy ORIGIN_SECRET=ci-origin-secret-not-for-deploy npm run test:runtime
• EDGE_ADMIN_TOKEN=ci-build-token-not-for-deploy ORIGIN_SECRET=ci-origin-secret-not-for-deploy npm run test:drift