add note about makefile being authoritative docs

prompts/REPO_POLICIES.md

@@ -43,6 +43,16 @@ style conventions are in separate documents:
   instead of invoking the underlying tools directly. The Makefile is the single
   source of truth for how these operations are run.
 
+- The Makefile is authoritative documentation for how the repo is used. Beyond
+  the required targets above, it should have targets for every common operation:
+  running a local development server (`make run`, `make dev`), re-initializing
+  or migrating the database (`make db-reset`, `make migrate`), building
+  artifacts (`make build`), generating code, seeding data, or anything else a
+  developer would do regularly. If someone checks out the repo and types
+  `make<tab>`, they should see every meaningful operation available. A new
+  contributor should be able to understand the entire development workflow by
+  reading the Makefile.
+
 - Every repo should have a `Dockerfile`. All Dockerfiles must run `make check`
   as a build step so the build fails if the branch is not green. For non-server
   repos, the Dockerfile should bring up a development environment and run