feat: add functions, control structures, and clean code checklist from Clean Code course

[ohmji]

Summary

• Add Functions & Methods section to clean-code.md covering parameter
  minimization, abstraction levels, reasonable splitting, and side effects
• Add Control Structures section with guards/fail fast, deep nesting
  avoidance, polymorphism/factory patterns, and error handling best practices
• Add Classes & Objects section distinguishing objects vs data containers
  and class cohesion
• Add Clean Code Checklist as a practical quick-reference for naming,
  formatting, functions, control structures, and classes
• Update SKILL.md with new function and control structure guidelines
• Add credit for Academind GmbH / Maximilian Schwarzmuller (2020)

Why

The existing clean-code.md covered naming and object calisthenics well but
was missing practical guidance on writing clean functions (parameters, size,
abstraction levels, side effects) and control structures (guards, nesting,
error handling) — key topics from the Clean Code course summary PDF.