Prepare v4.0.0 release

- Rewrite release notes centering new features (two-stage review,
  debugging tools, test infrastructure)
- Bump version to 4.0.0 in plugin.json and marketplace.json
- Remove .claude/settings.local.json from tracking
- Add .claude/ to .gitignore

.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": "3.6.2",
+      "version": "4.0.0",
       "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": "3.6.2",
+  "version": "4.0.0",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"