fix: resolve CodiMD startup warnings

- Fix health check endpoint from /healthz to / (root path)
- Add CMD_DOMAIN environment variable to resolve domain warning
- Add CMD_URL_ADDPORT and CMD_PROTOCOL_USESSL configuration
- Update .env.example with domain configuration examples

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

.env.example

@@ -6,5 +6,14 @@ POSTGRES_DB=codimd
 # CodiMD Configuration
 CMD_DB_URL=postgres://codimd:CHANGE_THIS_TO_A_STRONG_PASSWORD@database/codimd
 
+# CodiMD Domain Configuration
+# For local development, use localhost
+# For production, set your actual domain (e.g., hackmd.example.com)
+CMD_DOMAIN=localhost
+
+# URL Configuration
+CMD_URL_ADDPORT=false
+CMD_PROTOCOL_USESSL=false
+
 # CodiMD Session Secret (generate a random string)
 CMD_SESSION_SECRET=CHANGE_THIS_TO_A_RANDOM_SECRET_STRING