Unified SecretStore for Environment and Keyring resolution

[cdvelop]

This change introduces a unified SecretStore component that centralizes credential management. It resolves secrets by checking environment variables first and falling back to the system keyring only if necessary. This allows devflow tools (gorelease, gopush, codejob) to run in CI/CD and headless environments without user interaction, while maintaining the current interactive experience for local development.

Key changes:

• New secret_store.go with SecretStore implementation and thread-safe lazy keyring initialization.
• Migration of github_auth.go and codejob_auth.go to use the new store.
• Updated docs/GORELEASE.md, docs/CODEJOB.md, and docs/github/diagrams/GITHUB_AUTH_FLOW.md with CI/CD usage instructions and updated flow diagrams.
• New tests in test/secret_store_test.go verifying the precedence and lazy initialization logic.

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PR created automatically by Jules for task 10880722830233109775 started by @cdvelop

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