refactor: Implement YAML-based difficulty and time system

- Added educational metadata (difficulty, time_estimate) to all module.yaml files
- Updated convert_readmes.py to read from YAML instead of hardcoded mappings
- Standardized difficulty progression: ⭐ → ⭐⭐ → ⭐⭐⭐ → ⭐⭐⭐⭐ → ⭐⭐⭐⭐⭐🥷
- Fixed path resolution for YAML reading in book build process
- Eliminated duplication: single source of truth for educational metadata
- Capstone gets special ninja treatment (⭐⭐⭐⭐⭐🥷) as beyond-expert level

modules/source/01_setup/module.yaml

@@ -19,6 +19,10 @@ files:
   readme: "README.md"
   tests: "inline"
 
+# Educational Metadata
+difficulty: "⭐"
+time_estimate: "1-2 hours"
+
 # Components - What's implemented in this module
 components:
   - "personal_info"