[codex] Document optional signing key id

[wk1995]

Summary

• Document SIGNING_KEY_ID as optional for PublishPlugin Central release setup.
• Keep the signing key id normalization fallback covered by workflow/script tests.

Validation

• python3 .github/scripts/ensure_publish_version_test.py
• python3 .github/scripts/normalize_signing_key_id_test.py
• python3 .github/scripts/sync_readme_publish_version_test.py
• python3 .github/scripts/publish_plugin_central_workflow_test.py
• ./gradlew :plugin_base:build --stacktrace
• ./gradlew :plugin_base:publishToMavenLocal --stacktrace
• python3 .github/scripts/validate_publish_plugin_publications.py
• python3 .github/scripts/sync_readme_publish_version.py --check
• ./gradlew :plugin_base:publishAllPublicationsToCentralStagingRepository -PcentralPublish=true --dry-run --stacktrace