fix: Remove fake HAPPY_CODER_API_KEY, use actual Happy Coder env vars

Happy Coder does not require an API key. It uses these environment variables:
- HAPPY_SERVER_URL (optional custom server)
- HAPPY_WEBAPP_URL (optional custom webapp)
- HAPPY_HOME_DIR (data directory)
- HAPPY_EXPERIMENTAL (enable experimental features)

Changes:
- Remove HAPPY_CODER_API_KEY from all files
- Add proper Happy Coder environment variables to StatefulSet
- Add Happy Coder config options to ConfigMap
- Update README with correct Happy Coder configuration
- Update Makefile and docker-compose examples
- Update secrets-example.yaml

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

k8s/statefulset.yaml

@@ -77,13 +77,23 @@ spec:
               name: antigravity
               key: github-token
               optional: true
-        # Happy Coder configuration
-        - name: HAPPY_CODER_API_KEY
+        # Happy Coder configuration (optional)
+        - name: HAPPY_SERVER_URL
           valueFrom:
-            secretKeyRef:
+            configMapKeyRef:
               name: antigravity
-              key: happy-coder-api-key
+              key: happy-server-url
               optional: true
+        - name: HAPPY_WEBAPP_URL
+          valueFrom:
+            configMapKeyRef:
+              name: antigravity
+              key: happy-webapp-url
+              optional: true
+        - name: HAPPY_HOME_DIR
+          value: "/home/claude/.happy"
+        - name: HAPPY_EXPERIMENTAL
+          value: "true"
         resources:
           requests:
             memory: "2Gi"