Correct the kept-lines bullet; test .editorconfig with no own line

REPO_POLICIES said an ignore file lacking the closing comment keeps every
line the canonical file has never held; blank lines and comments are
dropped, so it now says entries, in a bullet of its own.

A new scenario covers an .editorconfig lacking the comment with no line
of its own, which the script replaces.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -951,9 +951,9 @@ style conventions are in separate documents:
     files is at the top of `script/install`. The script replaces each file and
     keeps only the repo's own entries:
     - in `.gitignore`, `.editorconfig` and `.dockerignore`: the lines below the
-      comment that ends the canonical file, and, in a `.gitignore` or
-      `.dockerignore` lacking that comment, every line the canonical file has
-      never held
+      comment that ends the canonical file
+    - in a `.gitignore` or `.dockerignore` lacking that comment: every entry the
+      canonical file has never held
     - in `.golangci.yml`: the entries the repo added to the `deny` list of the
       `test-support` depguard rule