Add Cursor plugin manifest and hook response compatibility

Enable native Cursor plugin discovery with a .cursor-plugin manifest, and make the SessionStart hook emit both Cursor and Claude response shapes so context injection works across both platforms. Document Cursor install usage in the README while keeping Claude-first wording.

Co-authored-by: Cursor <cursoragent@cursor.com>

.cursor-plugin/plugin.json

@@ -0,0 +1,18 @@
+{
+  "name": "superpowers",
+  "displayName": "Superpowers",
+  "description": "Core skills library: TDD, debugging, collaboration patterns, and proven techniques",
+  "version": "4.3.0",
+  "author": {
+    "name": "Jesse Vincent",
+    "email": "jesse@fsck.com"
+  },
+  "homepage": "https://github.com/obra/superpowers",
+  "repository": "https://github.com/obra/superpowers",
+  "license": "MIT",
+  "keywords": ["skills", "tdd", "debugging", "collaboration", "best-practices", "workflows"],
+  "skills": "./skills/",
+  "agents": "./agents/",
+  "commands": "./commands/",
+  "hooks": "./hooks/hooks.json"
+}