prepare repo for public release

Add MIT LICENSE file, CONTRIBUTING.md with skill quality guidelines,
expand .gitignore for safety, and remove leftover idea-refine.zip artifact.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CONTRIBUTING.md

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+# Contributing to Agent Skills
+
+Thanks for your interest in contributing! This project is a collection of production-grade engineering skills for AI coding agents.
+
+## Adding a New Skill
+
+1. Create a directory under `skills/` with a kebab-case name
+2. Add a `SKILL.md` following the format in [docs/skill-anatomy.md](docs/skill-anatomy.md)
+3. Include YAML frontmatter with `name` and `description` fields
+4. Ensure the `description` starts with "Use when" and describes triggering conditions
+
+### Skill Quality Bar
+
+Skills should be:
+
+- **Specific** — Actionable steps, not vague advice
+- **Verifiable** — Clear exit criteria with evidence requirements
+- **Battle-tested** — Based on real engineering workflows, not theoretical ideals
+- **Minimal** — Only the content needed to guide the agent correctly
+
+### Structure
+
+Every skill should include these sections:
+
+- **Overview** — What this skill does and why it matters
+- **When to Use** — Triggering conditions
+- **Process** — Step-by-step workflow
+- **Common Rationalizations** — Excuses agents use to skip steps, with rebuttals
+- **Red Flags** — Warning signs that the skill is being applied incorrectly
+- **Verification** — How to confirm the skill was applied correctly
+
+### What Not to Do
+
+- Don't duplicate content between skills — reference other skills instead
+- Don't add skills that are vague advice instead of actionable processes
+- Don't create supporting files unless content exceeds 100 lines
+- Don't put reference material inside skill directories — use `references/` instead
+
+## Modifying Existing Skills
+
+- Keep changes focused and minimal
+- Preserve the existing structure and tone
+- Test that YAML frontmatter remains valid after edits
+
+## Reporting Issues
+
+Open an issue if you find:
+
+- A skill that gives incorrect or outdated guidance
+- Missing coverage for a common engineering workflow
+- Inconsistencies between skills
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the MIT License.