script/install-precommit: work where .git is a file (closes #129)

The script wrote the hook to .git/hooks, which fails when .git is a
file rather than a directory, as in a clone made with
--separate-git-dir. It now asks git for the repository's own git
directory with `git rev-parse --git-common-dir`, creates its hooks
directory if missing, and writes the hook there. In an ordinary clone
that is .git/hooks, so nothing moves. git's core.hooksPath setting is
not followed, so the script never writes outside the repository; where
that setting is in force, git does not run the installed hook, as
before.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-10-01: `script/install-precommit` asks git for the repository's git
+  directory, so `make hooks` also works where `.git` is a file (closes #129).
 - 2026-09-29: the live-DNS test package is renamed `internal/livednstest` and
   added to the `test-support` `deny` list in `.golangci.yml`, so `make lint`
   fails when program code imports it (closes #164).