Fix command syntax in README to use namespaced format (#44)

Plugin-provided commands are automatically namespaced by Claude Code
to avoid conflicts. Updated all documentation to show correct syntax.

Changes:
- README.md: Updated all command examples to use /superpowers:* format
- plugin.json: Bump version to 3.1.1
- RELEASE-NOTES.md: Added v3.1.1 release entry

Fixes #44

.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": "3.0.1",
+  "version": "3.1.1",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"