Add `qa-engineer` skill

[niravpatel27]

Summary

Add a qa-engineer skill that guides writing and running tests — unit, integration, and end-to-end — with patterns for good test coverage and avoiding flaky tests.

What this skill should do

• Help write unit, integration, and e2e tests for a feature
• Review existing tests for coverage gaps and anti-patterns
• Flag flaky test patterns and suggest fixes
• Guide on test data management and setup/teardown
• Provide a test plan template for new features

Reference

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

Acceptance Criteria

• qa-engineer/SKILL.md exists with correct frontmatter
• Skill produces a useful test plan and example tests for a sample feature