Variable discovery and validation improvements #94

Summary
Template variables are discovered via Jinja parsing and prompted, but type/validation enforcement is minimal.

Proposed changes

• Support types (string, number, boolean) from config variables and coerce/validate inputs.
• Add validators: regex, min/max, enum.
• Add default-from-env and required flags.
• Add --vars-file path.yaml to supply variables as YAML (parallel to --mappings-file).

Files

• struct_module/template_renderer.py
• struct_module/commands/generate.py
• docs/template-variables.md, docs/configuration.md

Acceptance criteria

• Variables are validated/coerced according to type and constraints.
• --vars-file works and merges with --vars.
• Clear error messages when validation fails; tests included.