WIP: Gemini CLI extension infrastructure

Add experimental Gemini CLI extension with MCP server that exposes
skills as individual tools. Infrastructure works but auto-triggering
skills is blocked by Gemini CLI treating context files as advisory
rather than executable instructions.

See issue #128 for detailed findings.

- gemini-extension.json manifest
- MCP server with individual skill tools
- GEMINI.md bootstrap attempts (don't work)
- Installation documentation

.gemini/mcp-server/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "superpowers-gemini-mcp-server",
+  "version": "3.5.1",
+  "type": "module",
+  "main": "dist/superpowers.js",
+  "scripts": {
+    "build": "tsc",
+    "dev": "tsc --watch",
+    "start": "node dist/superpowers.js",
+    "typecheck": "tsc --noEmit",
+    "prepare": "npm run build"
+  },
+  "devDependencies": {
+    "@types/node": "^22.0.0",
+    "typescript": "^5.0.0"
+  },
+  "dependencies": {
+    "@modelcontextprotocol/sdk": "^1.18.0",
+    "zod": "^3.24.2"
+  }
+}