Clarify that Skill tool loads skill content directly

Fixes pattern where Claude would invoke a skill then try to Read the
file separately. The Skill tool already provides the content.

- Add "How to Access Skills" section to using-superpowers
- Change "read the skill" → "invoke the skill"
- Commands now use fully qualified names (superpowers:brainstorming etc)

.claude-plugin/marketplace.json

@@ -9,7 +9,7 @@
     {
       "name": "superpowers",
       "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-      "version": "4.0.0",
+      "version": "4.0.1",
       "source": "./",
       "author": {
         "name": "Jesse Vincent",

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "superpowers",
   "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-  "version": "4.0.0",
+  "version": "4.0.1",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"