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

prompts/REPO_POLICIES.md

@@ -943,12 +943,14 @@ style conventions are in separate documents:
   the files that changed and does not push, so a person reviews and pushes the
   commit.
 
-    `script/install` fetches each vendored file the repo carries from `prompts`
-    over HTTP. It also installs `script/install` and `script/update-policy` in
-    the repo, so a repo updated once can update itself. Every repo made from a
-    template repo already has both. The list of vendored files is at the top of
-    `script/install`. The script replaces each file and keeps only the repo's
-    own entries:
+    `script/install` fetches each vendored file from `prompts` over HTTP and
+    writes it into the repo. It skips a language's files when git tracks no
+    source file of that language. A file the repo lacks is written whole. A
+    `prompts/NAME.md` goes to `docs/NAME.md`, unless the repo keeps it at
+    `NAME.md`. It also installs `script/install` and `script/update-policy` in
+    the repo, so a repo updated once can update itself. The list of vendored
+    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
     - in `.golangci.yml`: the entries the repo added to the `deny` list of the