SkillCheck validator, Cursor exports, and per-agent installers (#27)

Three more learnings from alirezarezvani/claude-skills, applied:

1. SkillCheck validator (scripts/skillcheck.mjs) — validates every SKILL.md
   against the authoring standard (frontmatter, name/folder match, trigger +
   produces clauses, required headings) plus tier referential integrity.
   Errors fail CI; --strict fails on warnings too. New skillcheck.yml workflow
   and a SkillCheck status badge in the README. Current: 0 errors / 14 advisory
   warnings across 172 skills.

2. Cursor export platform — build-exports.mjs now generates
   exports/cursor/<bundle>/<skill>/<skill>.mdc rule files. The PLATFORMS
   registry now supports per-skill filenames (file as a function).

3. Per-agent installers — scripts/install.sh unifies install for
   claude/hermes/codex/openclaw/cursor (--link, --target, --dry-run, --list).
   Curl-able one-liners codex-install.sh, openclaw-install.sh, and
   cursor-install.sh clone the library and install in a single command.

README documents the one-line installs and Cursor exports; CHANGELOG and the
authoring standard updated.


Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

Co-authored-by: Claude <noreply@anthropic.com>

SKILL-AUTHORING-STANDARD.md

@@ -74,6 +74,8 @@ A skill is ready to merge when:
       trustworthy, not just a prompt.
 - [ ] It works with no setup beyond reading the file (scripts excepted, and those are
       stdlib-only).
+- [ ] It passes **SkillCheck**: `node scripts/skillcheck.mjs` reports no errors (warnings
+      are advisory). CI runs this on every PR that touches `skills/`.
 
 ## 5. Helper scripts (optional)