AGENTS.md: how to run and restart the dev stack

Ports, start/restart commands, the kill-by-PID gotcha and the test command
so a fresh session can bring the environment back without guessing.

AGENTS.md

@@ -40,6 +40,18 @@ Project constraints (do not regress):
   GIT_HASH baked in for the version pill).
 - Security/permission tests live in backend/apps/core/tests and need a
   one-time grant: GRANT ALL ON `test_j621`.* TO 'j621'@'%';
+
+Dev environment (this machine):
+- MariaDB/Redis run from the repo root: `docker compose up -d` (host ports
+  3307 and 6380 — other local projects own 3305/6379/3406).
+- Backend dev server: `backend/venv/bin/python manage.py runserver 0.0.0.0:8000`.
+  Django's system checks touch the database, so start MariaDB first; run it
+  from backend/ with the venv python.
+- Frontend dev server: `npm run dev` in frontend/ — Vite listens on
+  [::1]:5173 (http://localhost:5173) and proxies /api to 127.0.0.1:8000.
+- Restarting: kill dev servers by PID from `pgrep -af`; never
+  `pkill -f "manage.py runserver"` (the pattern matches its own shell).
+- Suite: `backend/venv/bin/python manage.py test apps.core.tests` (~80 s).
 - Same-origin and cross-origin frontends both work, and the SPA can also run
   with no backend at all: a production build asks on first start (/setup) and
   stores the answer in localStorage — a URL, "" for same origin, or "none"