Add test requirements to checklists, .dockerignore URLs, root symlink, and Makefile comment

- Add test requirement item to both checklists (must not be a no-op)
- Add .dockerignore template URL to Dockerfile items in both checklists
- Add REPO_POLICIES.md symlink in repo root pointing to prompts/
- Add comment to Makefile explaining why prettier flags are repeated

prompts/EXISTING_REPO_CHECKLIST.md

@@ -30,7 +30,8 @@ with your task.
 - [ ] `.editorconfig` exists — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.editorconfig`
 - [ ] `Dockerfile` and `.dockerignore` exist; Dockerfile runs `make check` as a
-      build step
+      build step — fetch `.dockerignore` from
+      `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore`
 - [ ] Gitea Actions workflow in `.gitea/workflows/` runs `docker build .` on
       push — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`
@@ -52,6 +53,8 @@ with your task.
       `hooks`
 - [ ] `make check` does not modify any files in the repo
 - [ ] `make test` has a 30-second timeout
+- [ ] `make test` runs real tests, not a no-op (at minimum, import/compile
+      check)
 - [ ] `make check` passes on current branch
 
 # Formatting