chore: add repo essentials and GitHub templates

- Add MIT LICENSE file
- Add CONTRIBUTING.md with skill creation guide
- Add AGENTS.md and CLAUDE.md for AI agent guidelines
- Add issue template for skill requests
- Add PR templates for new skills, updates, and docs
- Update README with links to contributing guide and license

.github/PULL_REQUEST_TEMPLATE/documentation.md

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+## Documentation
+
+## Summary
+
+<!-- What documentation changes are you making? -->
+
+## Files changed
+
+<!-- List the files you're updating -->
+
+## Checklist
+
+- [ ] Links are valid
+- [ ] Formatting is consistent with existing docs
+- [ ] No sensitive data or credentials

.github/PULL_REQUEST_TEMPLATE/new-skill.md

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+## New Skill
+
+**Skill name:** `skills/SKILL-NAME`
+
+## Summary
+
+<!-- What does this skill do and when should it be used? -->
+
+## Checklist
+
+- [ ] `name` matches directory name exactly
+- [ ] `name` follows naming rules (lowercase, hyphens, no `--`)
+- [ ] `description` is 1-1024 chars with trigger phrases
+- [ ] `SKILL.md` is under 500 lines
+- [ ] No sensitive data or credentials
+- [ ] Tested locally with AI agent

.github/PULL_REQUEST_TEMPLATE/skill-update.md

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+## Skill Update
+
+**Skill:** `skills/SKILL-NAME`
+
+## Summary
+
+<!-- What changes are you making and why? -->
+
+## Type of update
+
+- [ ] Bug fix
+- [ ] Improved instructions
+- [ ] Added references/scripts
+- [ ] Other
+
+## Checklist
+
+- [ ] Changes are focused and minimal
+- [ ] `SKILL.md` is still under 500 lines
+- [ ] No sensitive data or credentials
+- [ ] Tested locally with AI agent