[codex] Configure Android release signing

[jjoonleo]

Summary

• Configure Android release builds to use a real release signing config instead of debug signing.
• Load signing inputs from android/key.properties, ANDROID_* environment variables, or the existing ONTIME_ANDROID_* variables from the production metadata branch.
• Add Android release signing documentation and update release checklist/docs index entries.

Why

Fixes #385. Release APK/AAB artifacts must not be signed with debug keys, and missing signing inputs should fail clearly before production artifacts are produced.

Notes

• This branch includes the merged codexd/issue-396-production-metadata changes as requested.
• Debug/profile builds do not require release signing credentials.

Validation

• flutter build apk --debug
• flutter build appbundle --release without signing inputs fails with the expected signing error
• flutter build appbundle --release with a temporary JKS keystore and ANDROID_* env vars succeeds
• flutter analyze

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