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

README.md

@@ -105,6 +105,7 @@ 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.
+- **Google Gemini** — copy any [`exports/gemini/<bundle>/<skill>/GEM_INSTRUCTIONS.md`](exports/gemini/) into a Gem'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: