chore: add .gitignore and env template, untrack generated files

- Add .gitignore excluding generated docker-compose.yml, demo.env,
  editor files, and temporary files
- Remove demo/docker-compose.yml from tracking (generated by envsubst)
- Remove demo/demo.env from tracking (contains per-machine values)
- Add demo/demo.env.template as reference for required configuration
- Remove stale config/portainer/ directory (Portainer not in stack)

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

.gitignore

@@ -0,0 +1,21 @@
+# Generated files
+demo/docker-compose.yml
+
+# Environment with secrets
+demo/demo.env
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Editor files
+*.swp
+*.swo
+*~
+.vscode/
+.idea/
+
+# Temporary files
+*.tmp
+*.bak
+tmp_template.yml