Support agent secrets and variables

[MariusStorhaug]

Summary

Add support for using agent-level secrets and variables in Process-PSModule workflows/actions.

Problem

Current workflow usage does not provide a clear/native way to consume agent secrets and variables, which limits secure and reusable automation setup.

Proposal

Implement support so Process-PSModule can read and use:

• Agent secrets
• Agent variables

Expected behavior

1. Secrets are consumable without exposing values in logs.
2. Variables are available as normal configuration inputs.
3. Documentation includes a minimal example for both secrets and variables.

Acceptance criteria

• Agent secrets are resolved and masked correctly.
• Agent variables are resolved consistently across workflow steps.
• Backward compatibility is maintained for existing configurations.