Restructure prompt-engineering.mdc for better flow and coherence

Major improvements:
- Remove redundant "Core Principles" and "Optimization Techniques" sections
- Move "Structural Delimiters" section earlier (before examples use XML extensively)
- Promote Few-Shot and Token Efficiency to main sections
- Add cross-references between related sections
- Remove all excessive markdown formatting (bold, decorative symbols)
- Minimize "bad" examples per document's own advice
- Add meta section explaining minimal formatting philosophy

Better structure flow:
Philosophy → Principles → Core Concepts → Structure → Techniques → Advanced → Pitfalls

Fixes issues where XML tags were used extensively before being explained,
and eliminates teaching-before-introducing ordering problems.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: Nick Sullivan