Add `architecture` skill

[niravpatel27]

Summary

Add an architecture skill that helps design systems and make architectural decisions — covering trade-offs, patterns, diagrams, and documentation for new features or refactors.

What this skill should do

• Help evaluate architectural options with clear trade-offs
• Suggest appropriate design patterns (event-driven, microservices, etc.)
• Guide on how to document architectural decisions (ADRs)
• Help create system diagrams (Mermaid or text-based)
• Flag common architectural mistakes early

Reference

See existing skills like lfx-product-architect/SKILL.md for pattern and format.
Each skill lives in its own directory with a SKILL.md file and markdown instructions.

Acceptance Criteria

• architecture/SKILL.md exists with correct frontmatter
• Skill produces a useful architectural recommendation on a sample design question