feat: built-in shift-left QA skills

[rubenmarcus]

Summary

Create bundled skills for Shift-Left QA workflows, installable via ralph-starter skill add or included as built-in skills.

Skills to Create

1. spec-linter skill

Validates specs before coding. Checks for acceptance criteria, edge cases, definition of done.

2. acceptance-criteria skill

Extracts Given/When/Then from natural language specs. Generates Gherkin-format acceptance criteria.

3. test-first skill

Enforces TDD workflow: write failing tests first, then implement, then refactor.

Skill Composition

Allow skills to declare dependencies:

---
name: shift-left
description: Complete Shift-Left QA workflow
requires:
  - spec-linter
  - acceptance-criteria
  - test-first
---

This enables skill chains: spec-linter → acceptance-criteria → test-first → implementation.

Implementation

• Add requires field to skill frontmatter parsing in src/loop/skills.ts
• Create skill files in a bundled/installable format
• Expand src/commands/skill.ts registry with QA category