fix: use native fetch instead of node-fetch dependency

- Removed unnecessary node-fetch import from bin/brainy.js
- Removed node-fetch import from brainy-mcp-server.js
- Native fetch available since Node 18, brainy requires Node 24.4.0+
- Reduces dependencies by 6 packages
- Avoids indirect deprecation warning from boolean package
- No impact on browser compatibility (CLI is Node-only, core library unaffected)

Architecture notes:
- Brain Cloud integration remains as augmentation option
- Users can choose: local (free), Brain Cloud (9/mo), or self-host
- Clean separation maintained: brainy (MIT) vs Brain Cloud (proprietary)
- TODO: Consider extracting Brain Cloud specific URLs to config

package.json

@@ -201,7 +201,6 @@
     "express": "^5.1.0",
     "happy-dom": "^18.0.1",
     "jsdom": "^26.1.0",
-    "node-fetch": "^3.3.2",
     "process": "^0.11.10",
     "puppeteer": "^22.15.0",
     "standard-version": "^9.5.0",
@@ -220,7 +219,6 @@
     "commander": "^11.1.0",
     "dotenv": "^16.4.5",
     "inquirer": "^12.9.1",
-    "node-fetch": "^3.3.2",
     "ora": "^8.0.1",
     "prompts": "^2.4.2",
     "uuid": "^9.0.1"