script/install: write every listed file the repo lacks

A listed file the repo lacks is now written whole, with no keep rule.
A prompts/NAME.md goes to docs/NAME.md unless the repo keeps it at
NAME.md, and any other file goes to its own path. A language's files
are skipped when git tracks no source file of that language.

A listed path git ignores now stops the script before it writes.

README bullets split into short sentences, and a false sentence about
template repos deleted.

Model: opus-5-5

README.md

@@ -142,11 +142,11 @@ alpine. We provide:
 - `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 the repo given as its argument, or its own repo with no
-  argument (our own extension); it stops when run here
+  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 run here.
 - `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 and stops when run here
+  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`.