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/gemini-extension.json

@@ -0,0 +1,13 @@
+{
+  "name": "superpowers",
+  "description": "Core skills library: TDD, debugging, collaboration patterns, and proven techniques",
+  "version": "3.5.1",
+  "contextFileName": "GEMINI.md",
+  "mcpServers": {
+    "superpowers": {
+      "command": "node",
+      "args": ["${extensionPath}${/}mcp-server${/}dist${/}superpowers.js"],
+      "cwd": "${extensionPath}"
+    }
+  }
+}