feat: remove Happy Coder and Node.js from devcontainer

Happy Coder is no longer used. Node.js was only installed as a
dependency for `npm install -g happy-coder`, so both are removed.
This shrinks the Docker image and simplifies the configuration.

Removed from: Dockerfile, Helm values/schema/templates, serverless
manifests, Makefile, and all documentation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

DEPLOYMENT.md

@@ -225,18 +225,6 @@ spec:
 
 ## Advanced Configurations
 
-### Custom Happy Coder Endpoints
-
-For self-hosted Happy instances:
-
-```bash
-helm install mydev ./chart \
-  --set name=mydev \
-  --set githubRepo=https://github.com/youruser/yourrepo \
-  --set happyServerUrl=https://your-happy-server.com \
-  --set happyWebappUrl=https://your-happy-webapp.com
-```
-
 ### Custom Display Resolution
 
 ```bash