Function build_context 

pub fn build_context(spec_id: &str, specs_dir: &Path) -> DerivationContext

Build a DerivationContext populated with current environment data.

This creates a fully populated context with:

• Current git branch name
• Spec file path (constructed from spec_id and specs_dir)
• All environment variables
• Git user name and email from config

This is the canonical way to build a context for derivation operations.