Move superpowers plugin into plugins/superpowers-bootstrap subdirectory

This restructures the repo to support multiple plugins by moving the current
plugin into a subdirectory. The plugin continues to be published as "superpowers"
in the marketplace.

hooks/hooks.json

@@ -1,15 +0,0 @@
-{
-  "hooks": {
-    "SessionStart": [
-      {
-        "matcher": "startup|resume|clear|compact",
-        "hooks": [
-          {
-            "type": "command",
-            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh"
-          }
-        ]
-      }
-    ]
-  }
-}