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

CHANGELOG.md

@@ -10,6 +10,12 @@ each new wave of skills bumps the **major** version, extensions and fixes bump
 ## [Unreleased]
 
 ### Added
+- **Multi-platform export generator.** `scripts/build-exports.mjs` renders every skill
+  into platform-ready files under `exports/` from a single source of truth (the
+  `SKILL.md` body), so content is never maintained twice. Ships ChatGPT
+  (`exports/chatgpt/<bundle>/<skill>/SYSTEM_PROMPT.md`) and a `PLATFORMS` registry that
+  makes adding Gemini/Cursor/etc. a few lines. Includes a `--check` mode and a
+  `check-generated` CI workflow that fails if exports or `web/skills.json` drift.
 - **Programmatic helpers (stdlib Python) for three flagship skills.** Each runs with
   zero dependencies and computes part of the work instead of estimating by hand:
   - `sprint-planning/scripts/capacity_calculator.py` — recommended sprint commitment