Document hot-reload workflow for AI agents

Rewrite SAAC_DEPLOYMENT.md to lead with the two-domain model:
- Production (yourapp.<server>.domain.com) — updates on saac deploy
- Hot-reload (yourapp-hot.<server>.domain.com) — auto-rebuilds on git push

Added: recommended dev workflow (push → check hot → deploy to prod),
nodemon.json explanation, development cycle diagram, customization guide.

Updated Dockerfile and docker-compose.yml headers to explain which
container uses which file and reference nodemon.json.

docker-compose.yml

@@ -1,6 +1,10 @@
-# SAAC Application Stack
+# SAAC Application Stack — PRODUCTION container
 # See SAAC_DEPLOYMENT.md for full deployment rules and common mistakes.
 #
+# This file is used by "saac deploy" to build the production Docker image.
+# Your app also gets a HOT-RELOAD container (yourapp-hot.<server>.domain.com)
+# that auto-rebuilds on every git push — see nodemon.json for its config.
+#
 # Key rules:
 #   1. Use "expose", NEVER "ports" — Traefik handles routing
 #   2. Database host = service name ("postgres"), NEVER "localhost"