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

SKILL-AUTHORING-STANDARD.md

@@ -24,6 +24,9 @@ skills/
 - Use lowercase, hyphenated names (`customer-journey-map`, not `CustomerJourneyMap`).
 - A skill must be useful with `SKILL.md` alone. Scripts are an enhancement, never a
   prerequisite.
+- **Never hand-edit `exports/`.** Those platform files (e.g. ChatGPT `SYSTEM_PROMPT.md`)
+  are generated from the `SKILL.md` body by `scripts/build-exports.mjs`. Edit the source
+  skill and regenerate; CI fails if they drift.
 
 ## 2. Frontmatter (required)