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>

Makefile

@@ -25,8 +25,8 @@ run:
 		-p 5800:5800 \
 		-e GITHUB_REPO="${GITHUB_REPO}" \
 		-e GITHUB_TOKEN="${GITHUB_TOKEN}" \
-		-e HAPPY_CODER_API_KEY="${HAPPY_CODER_API_KEY}" \
 		-e VNC_PASSWORD="${VNC_PASSWORD}" \
+		-e HAPPY_EXPERIMENTAL="true" \
 		-v $(PWD)/home:/home \
 		-v $(PWD)/workspace:/workspace \
 		--name antigravity \
@@ -93,7 +93,6 @@ help:
 	@echo "Environment Variables for 'make run':"
 	@echo "  GITHUB_REPO        - GitHub repository URL"
 	@echo "  GITHUB_TOKEN       - GitHub token (optional)"
-	@echo "  HAPPY_CODER_API_KEY - Happy Coder API key"
 	@echo "  VNC_PASSWORD       - VNC password (optional)"
 	@echo ""
 	@echo "Example:"