Bug: Adding variables breaks file fetching (github:// protocol) #183

Summary

Adding a variables section to a structure definition breaks file fetching, specifically with github:// protocol. The error is 'list' object is not a mapping.

Reproduction Steps

Case 1: Works fine (no variables)

description: "Worker Python asynchrone avec composition (project/python)."

files:
  - .gitignore:
      file: github://github/gitignore/main/Python.gitignore

✅ Generates correctly

Case 2: Breaks (with variables)

description: "Worker Python asynchrone avec composition (project/python)."

variables:
  - project_name:
      description: "Name of your project"
      type: string
      default: "MyProject"

files:
  - .gitignore:
      file: github://github/gitignore/main/Python.gitignore

❌ Error: Failed to fetch content from github://github/gitignore/main/Python.gitignore: 'list' object is not a mapping

Expected Behavior

• Variables and file fetching should work together
• The .gitignore file should be fetched from GitHub

Actual Behavior

• Adding variables section causes file fetch to fail
• Error message: 'list' object is not a mapping

Root Cause Analysis

The error 'list' object is not a mapping suggests:

• When parsing variables (which is a list: variables: [...]), something in the file fetching logic expects a dictionary
• The file content fetching is somehow processing the variables list instead of the file definition
• This could be a YAML parsing issue or an ordering bug in how files and variables are processed

Possible Fix Areas

• structkit/content_fetcher.py - verify it handles the full config structure correctly
• structkit/template_renderer.py - check variable resolution doesn't interfere with file fetching
• structkit/file_item.py - ensure file items are parsed correctly when variables are present
• structkit/commands/generate.py - check config loading/processing order

Environment

• StructKit version: 3.2.1
• Python version: 3.x
• OS: Linux (container)