feat: add .goscaf.yaml support for persistent global and local defaults

[KushalNaral]

Two scopes are supported, a global file at ~/.goscaf.yaml for personal machine defaults, and a local ./.goscaf.yaml that can be committed to a repo for team-wide standardisation. When both
exist, local values take precedence over global. CLI flags always win over both.

In interactive mode the config pre-fills prompt defaults, the user still sees every question and can change any answer. In flag-driven mode the config fills in any field not covered by an
explicit flag. The --defaults flag bypasses the config entirely and uses hardcoded values as before, keeping CI pipelines unaffected.

Fixes #18

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• make test
• make lint
• make smoke-test

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules