Rename script/install to script/fetch-policy

By convention, and in scripts-to-rule-them-all, script/install installs
the program itself. This script fetches the shared policy files into a
repo. The make target, the test names and the messages take the new
name. Behaviour is unchanged.

Model: opus-5-5

README.md

@@ -141,13 +141,13 @@ alpine. We provide:
   `script/check`
 - `script/install-precommit` — installs the git pre-commit hook (our own
   extension); `make hooks` shims to it
-- `script/install` — fetch the vendored policy files from this repo over HTTP
-  and write them into a repo (our own extension). It writes into the repo given
-  as its argument, or its own repo with no argument. It stops when its target is
-  this repo.
-- `script/update-policy` — fetch the current `script/install`, run it on its own
-  repo and commit the result without pushing (our own extension). It is for the
-  repos that vendor the policy files. It stops when run here.
+- `script/fetch-policy` — fetch the vendored policy files from this repo over
+  HTTP and write them into a repo (our own extension). It writes into the repo
+  given as its argument, or its own repo with no argument. It stops when its
+  target is this repo.
+- `script/update-policy` — fetch the current `script/fetch-policy`, run it on
+  its own repo and commit the result without pushing (our own extension). It is
+  for the repos that vendor the policy files. It stops when run here.
 
 `make hooks` installs the pre-commit hook that runs `script/precommit`.