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>

Makefile

@@ -26,7 +26,6 @@ run:
 		-e GITHUB_REPO="${GITHUB_REPO}" \
 		-e GITHUB_TOKEN="${GITHUB_TOKEN}" \
 		-e VNC_PASSWORD="${VNC_PASSWORD}" \
-		-e HAPPY_EXPERIMENTAL="true" \
 		-v $(PWD)/home:/home \
 		-v $(PWD)/workspace:/workspace \
 		--name devcontainer \