document process assumptions

ai/shared.md

@@ -20,6 +20,13 @@ guides (`backend-context.md`, `frontend-context.md`) extend these.
    batch multiple behaviors into one failing-test commit and one
    implementation commit when they can be reviewed separately.
 
+## Runtime assumptions
+
+- Assume every process defined in `process-compose.yml` is already running
+  for local development and verification unless a command proves otherwise.
+  Do not start duplicate PostgreSQL, backend, or frontend processes just to
+  run checks.
+
 ## Code style
 
 - Lines should not exceed 80 columns, but should use up to 80 columns when