Move Gemini extension to repo root for cross-platform support

Symlinks inside .gemini/ don't work on Windows. Moving
gemini-extension.json and GEMINI.md to the repo root means
the extension root IS the repo root, so skills/ is found
naturally without symlinks.

.gemini/GEMINI.md

@@ -1 +0,0 @@
-@../skills/using-superpowers/SKILL.md

.gemini/skills

@@ -1 +0,0 @@
-../skills

GEMINI.md

@@ -0,0 +1 @@
+@./skills/using-superpowers/SKILL.md