fix(security): make gitleaks-missing failure explicit + add Ubuntu apt guidance

When gitleaks is absent the installer now states outright that the
pre-commit hook was NOT installed (on stderr) and exits 1, and the Linux
guidance gains 'sudo apt install gitleaks' (works on Ubuntu 24.04)
alongside the existing releases pointer. Verified by execution: missing
gitleaks → exit 1, no hook written; gitleaks present → hook installed
and executable.

Author: fidgetcoding

scripts/install-pre-commit-hook.sh

@@ -7,9 +7,11 @@ REPO_ROOT="$(git rev-parse --show-toplevel)"
 HOOK="$REPO_ROOT/.git/hooks/pre-commit"
 
 if ! command -v gitleaks &>/dev/null; then
-  echo "❌ gitleaks not found on PATH."
-  echo "   macOS:  brew install gitleaks"
-  echo "   Linux:  https://github.com/gitleaks/gitleaks/releases"
+  echo "❌ gitleaks not found on PATH — pre-commit hook NOT installed." >&2
+  echo "   Install gitleaks, then re-run this script:" >&2
+  echo "   macOS:  brew install gitleaks" >&2
+  echo "   Linux:  sudo apt install gitleaks   (Ubuntu 24.04+)" >&2
+  echo "           or grab a release: https://github.com/gitleaks/gitleaks/releases" >&2
   exit 1
 fi