Drop make install from the required targets; test three more stops

The install target stays in this repo's Makefile only. sneak/vaultik
already has an install target with another meaning, and every repo
still gets script/install.

Tests: a repo's own script/update-policy stops the script, and a repo
whose only JavaScript source is a .js or .mjs file gets the JavaScript
style guide.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -93,7 +93,6 @@ style conventions are in separate documents:
     - `make check` (runs `test`, `lint`, `fmt-check`)
     - `make docker`
     - `make hooks` (installs pre-commit hook)
-    - `make install` (writes the current policy files into the repo)
     - `make update-policy` (brings the repo up to standards and commits)
 
     A model Makefile is at
@@ -194,7 +193,7 @@ style conventions are in separate documents:
       `make hooks` target shims to it.
     - `script/install` fetches the vendored policy files from the `prompts` repo
       over HTTP and writes them into the repo it is given, or its own repo with
-      no argument. The `make install` target shims to it.
+      no argument.
     - `script/update-policy` fetches the current `script/install` from the
       `prompts` repo, runs it on its own repo and commits the result. It does
       not push. The `make update-policy` target shims to it.