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

TODO.md

@@ -21,11 +21,11 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-11-09: `script/install` writes the vendored policy files into a repo over
-  HTTP, and installs itself and `script/update-policy` there
+- 2026-11-09: `script/fetch-policy` writes the vendored policy files into a repo
+  over HTTP, itself and `script/update-policy` among them
   (https://git.eeqj.de/sneak/prompts/issues/165). `make update-policy` now
-  fetches the current `script/install`, runs it and commits without pushing. The
-  template repositories get both scripts in their own issues.
+  fetches the current `script/fetch-policy`, runs it and commits without
+  pushing. The template repositories get both scripts in their own issues.
 - 2026-11-09: `script/update-policy` and `make update-policy` update a repo's
   vendored policy files with one command
   (https://git.eeqj.de/sneak/prompts/issues/162). They fetch the files from