fix: move obsidian to devDependencies, improve shim export

- package.json: move obsidian from dependencies to devDependencies
  (it is a type stub — having it in dependencies risks shadowing
  Obsidian's built-in API if node_modules is present in the plugin folder)
- main.js: explicitly extract default export from dist/main.js so the
  shim works with both plugin.default and direct-export loader patterns

package.json

@@ -27,12 +27,12 @@
     "eslint": "^8.56.0",
     "jest": "^29.7.0",
     "jest-environment-jsdom": "^30.3.0",
+    "obsidian": "^1.4.11",
     "prettier": "^3.2.5",
     "ts-jest": "^29.1.2",
     "typescript": "^5.3.3"
   },
   "dependencies": {
-    "chromadb": "^1.5.3",
-    "obsidian": "^1.4.11"
+    "chromadb": "^1.5.3"
   }
 }