script/install-precommit: write the hook through $hook (closes #139)

The script set hook and then repeated the path literally, so shellcheck
reported the variable as unused (SC2034).

Model: opus-5-5

script/install-precommit

@@ -8,8 +8,8 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
     hook=".git/hooks/pre-commit"
-    printf '#!/bin/sh\nset -e\nscript/precommit\n' > .git/hooks/pre-commit
+    printf '#!/bin/sh\nset -e\nscript/precommit\n' > "$hook"
-    chmod +x .git/hooks/pre-commit
+    chmod +x "$hook"
     echo "pre-commit hook installed: runs script/precommit"
 }