refactor: change default secret name to OPENCODE_AUTH

[activadee]

Summary

• Rename default environment variable to OPENCODE_AUTH throughout the codebase
• Update configuration schema and example files to use new default variable name
• Add GitHub Actions workflow for opencode-auth validation
• Update unit tests to verify new default variable behavior
• Update documentation in README with the new variable reference

[github-actions]

AI Review Summary

Verdict: APPROVE
Commit: ea8c008

Findings

No significant issues found.

Overall Assessment

Clean refactor that consistently renames the default secret name from OPENCODE_AUTH_JSON to OPENCODE_AUTH throughout the codebase. All changes are properly synchronized across code, tests, documentation, and schema files.