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

exports/README.md

@@ -0,0 +1,16 @@
+# Multi-Platform Exports
+
+These folders are **generated** from the canonical `skills/*/SKILL.md` files —
+the skill body is the single source of truth. Do not edit anything in `exports/`
+by hand; edit the source skill and run:
+
+```bash
+node scripts/build-exports.mjs
+```
+
+Currently exporting **172 skills** to:
+
+- **ChatGPT — Custom GPT instructions** → `exports/chatgpt/`
+
+Adding a new platform is a few lines in the `PLATFORMS` registry of
+`scripts/build-exports.mjs` — no content is duplicated.