feat(harness): /harness skill — guided harness engineering workflow

[magicpro97]

User Story

As a developer using GitHub Copilot CLI, I want a /harness skill that I can invoke to get guided harness engineering workflows, so that I can initialize, configure, and troubleshoot project harnesses without memorizing all the sk harness commands.

Acceptance Criteria

• .github/skills/harness/SKILL.md created with valid YAML frontmatter
• Skill description triggers on: "harness init", "setup harness", "harness engineering", "agent evaluation", "success criteria", "sk harness", "harness.yaml", "make agent work reproducible"
• Skill covers: Quick Start, Commands Reference, harness.yaml schema, Workflows (new project / retrofit / CI), Integration with sk toolchain (briefing, learn, tentacle, hooks)
• Skill deployed to ~/.copilot/skills/ via install.py --deploy-global-skills
• Skill under 200 lines

Priority: MEDIUM

Labels: enhancement, harness, priority:medium

Files: .github/skills/harness/SKILL.md

[magicpro97]

Implemented in PR #734. .github/skills/harness/SKILL.md created (160 lines, under 200 AC). Full guided workflow for sk harness init, check, doctor, config, show.