Install hooks by copying into .git/hooks instead of core.hooksPath

Linking core.hooksPath to the tracked, sandbox-writable .githooks/ let a
sandboxed command rewrite a hook that git commit/push then run unsandboxed.
Copies in .git/hooks can't be changed from the sandbox. This also keeps
existing .git/hooks scripts working and lets npm install succeed outside a
git checkout. pre-merge-commit now names itself in errors; hook tests clean
up, ignore GIT_* env and cover the installer; docs state what each layer
does and doesn't stop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.githooks/pre-commit

@@ -1,7 +1,10 @@
 #!/bin/sh
 # Refuse commits (and, via pre-merge-commit, merge commits) while main is checked out.
+# Cherry-pick, revert and fast-forward merges don't run these hooks; pre-push still stops
+# their result from reaching the remote.
+name=${1:-pre-commit}
 if [ "$(git symbolic-ref --short -q HEAD)" = "main" ]; then
-  echo "$(basename "$0"): committing on main is not allowed; create a branch first." >&2
+  echo "$name: committing on main is not allowed; create a branch first." >&2
   exit 1
 fi
 exit 0

.githooks/pre-merge-commit

@@ -1,2 +1,2 @@
 #!/bin/sh
-exec "$(dirname "$0")/pre-commit"
+exec sh "$(dirname "$0")/pre-commit" pre-merge-commit