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>

chart/values-dynamic.yaml

@@ -100,13 +100,6 @@ user:
 shm:
   sizeLimit: 2Gi
 
-# Happy Coder (ephemeral in dynamic mode)
-happy:
-  serverUrl: ""
-  webappUrl: ""
-  homeDir: "/tmp/.happy"  # Ephemeral location in dynamic mode
-  experimental: "true"
-
 # MCP sidecars are not supported in dynamic mode (Knative limitation)
 mcp:
   sidecars: