Add multi-platform export generator (single source of truth)

Make the library multi-platform without duplicating content. Each
skills/<name>/SKILL.md body remains the single source of truth; a new
generator renders platform-ready exports from it.

- scripts/build-exports.mjs — dependency-free Node generator with a PLATFORMS
  registry so new platforms (Gemini, Cursor, …) are a few lines. Ships ChatGPT
  exports at exports/chatgpt/<bundle>/<skill>/SYSTEM_PROMPT.md (172 skills),
  plus generated index READMEs. Supports --platform and --check.
- exports/ — generated ChatGPT system prompts, ready to paste into a Custom GPT.
- .github/workflows/check-generated.yml — fails a PR if exports or
  web/skills.json drift from the source skills.
- README "Works With" now documents the ready-to-use exports and regen command.
- CHANGELOG + SKILL-AUTHORING-STANDARD note the generated artifacts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

README.md

@@ -98,7 +98,23 @@ plugin install, automatic skill discovery from the `description`, and the helper
 invocation flow. On other tools you copy the body in manually and lose only the
 auto-triggering, not the substance.
 
-> Prefer ChatGPT? There's also a [companion Custom GPT library](#-companion-repository--chatgpt-custom-gpts) built from the same frameworks.
+### Ready-to-use exports
+
+You don't have to strip frontmatter by hand. Every skill is exported to platform-ready
+files under [`exports/`](exports/), generated from the same source so nothing is
+maintained twice:
+
+- **ChatGPT** — copy any [`exports/chatgpt/<bundle>/<skill>/SYSTEM_PROMPT.md`](exports/chatgpt/) straight into a Custom GPT's instructions.
+
+The skill body in `skills/<name>/SKILL.md` is the single source of truth. Regenerate (or
+add a new platform — it's a few lines in the `PLATFORMS` registry) with:
+
+```bash
+node scripts/build-exports.mjs            # regenerate all platform exports
+node scripts/build-exports.mjs --check    # CI: fail if exports are stale
+```
+
+> Prefer a hand-curated ChatGPT collection? There's also a [companion Custom GPT library](#-companion-repository--chatgpt-custom-gpts) built from the same frameworks.
 
 ---