feat: add text-to-diagram experimental skill

[jl681]

feat: add text-to-diagram decoder skill

• Implements architectural inference logic to automatically select the best
  visual representation (Sequence, C4, ERD, Flowchart, Mindmap) from text.
• Optimizes Mermaid.js syntax for high-fidelity rendering and token efficiency.
• Adds "Best Guess" strategy to reduce user friction during ambiguous requests.
• Provides a robust framework for transforming complex technical documentation
  into living architectural blueprints.