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>

serverless/deployment.yaml

@@ -155,11 +155,6 @@ spec:
           value: "1"
         - name: WEB_FILE_MANAGER_ALLOWED_PATHS
           value: "/workspace,/config"
-        # Happy Coder config (ephemeral in serverless mode)
-        - name: HAPPY_HOME_DIR
-          value: "/tmp/.happy"
-        - name: HAPPY_EXPERIMENTAL
-          value: "true"
         # Use secrets for sensitive data
         envFrom:
         - secretRef:

serverless/knative-service.yaml

@@ -49,11 +49,6 @@ spec:
           value: "1"
         - name: WEB_FILE_MANAGER_ALLOWED_PATHS
           value: "/workspace,/config"
-        # Happy Coder config
-        - name: HAPPY_HOME_DIR
-          value: "/config/userdata/.happy"
-        - name: HAPPY_EXPERIMENTAL
-          value: "true"
         # Use secrets for sensitive data
         envFrom:
         - secretRef: