Add Google Gemini exports as a second generated platform

Proves the PLATFORMS registry extends cleanly: adds Gemini (Gem instructions)
alongside ChatGPT, generated from the same SKILL.md source.

- scripts/build-exports.mjs: register `gemini` -> exports/gemini/<bundle>/<skill>/
  GEM_INSTRUCTIONS.md (body + a one-line role primer from the description).
- Fix: the root exports/README.md now always lists every registered platform,
  so `--platform x` no longer drops the others from the overview.
- exports/gemini/: 172 generated Gem instruction files + index.
- README "Ready-to-use exports" and CHANGELOG now list Gemini.

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

exports/README.md

@@ -11,6 +11,7 @@ node scripts/build-exports.mjs
 Currently exporting **172 skills** to:
 
 - **ChatGPT — Custom GPT instructions** → `exports/chatgpt/`
+- **Google Gemini — Gem instructions** → `exports/gemini/`
 
 Adding a new platform is a few lines in the `PLATFORMS` registry of
 `scripts/build-exports.mjs` — no content is duplicated.