Add design review skills and wire them into the review flow

[ahmedalaahagag]

What changed

This PR makes the design-review path in agent-skills explicit and self-contained.

It:

• adds two repo-native review skills:
  • lightweight-design-analysis
  • software-design-principles
• wires those skills into code-review-and-quality
• updates using-agent-skills so the discovery flow, lifecycle examples, and quick reference all include the new review skills
• updates the README skill catalog and count

Why

The existing review skill already covered architecture and readability, but the design-review path was implicit. This change makes that path explicit without changing the core five-axis review model.

The follow-up skill additions also make the PR self-contained: reviewers do not need extra local setup or separately installed skills for the new review path to make sense.

Scope

Intentionally included:

• skills/code-review-and-quality/SKILL.md
• skills/lightweight-design-analysis/SKILL.md
• skills/software-design-principles/SKILL.md
• skills/using-agent-skills/SKILL.md
• README.md

Intentionally excluded:

• /ship and persona changes
• marketplace/plugin wiring changes
• broader docs beyond the README catalog update

Validation

• verified the repo now contains both new skill files under skills/
• verified code-review-and-quality references both skills
• verified using-agent-skills discovery and quick reference mention both skills
• kept the work as focused commits on top of current main