fix: implement dynamic template loader for include functionality

[bordeux]

• Add 'loader' feature to minijinja dependency for dynamic template loading
• Implement lazy template loader using env.set_loader() in renderer
• Template loader uses TemplateContext to resolve relative paths
• Security: blocks absolute paths and parent traversal unless --trust mode
• Add comprehensive test suite with 12 integration tests covering:
  • Basic include functionality
  • Nested includes (multiple levels)
  • Include from subdirectories
  • Environment variables in included templates
  • Security restrictions (parent directory and absolute path blocking)
  • Trust mode allowing parent directory access
  • Conditional includes
  • Error handling for missing templates
• Add test fixtures for include templates and expected outputs
• All 267 tests passing (251 unit + 12 integration + 4 doc tests)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.91%. Comparing base (3e2e61d) to head (989feba).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   86.51%   86.91%   +0.39%     
==========================================
  Files          15       15              
  Lines         497      512      +15     
==========================================
+ Hits          430      445      +15     
  Misses         67       67              

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