feat: add uninstall scripts for X11 and macOS

scripts/uninstall-linux-x11.sh — strips ahfail from common PAM service
files and removes binaries (via ninja uninstall or known paths).

scripts/uninstall-macos.sh — strips ahfail from screensaverui/screensaver
and removes /usr/local/lib/ahfail/.

README: add Uninstall section covering all four cases (X11, Wayland,
Homebrew macOS, manual macOS).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/uninstall-macos.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# uninstall-macos.sh — remove ahfail from macOS (manual/script install)
+# For Homebrew installs: brew uninstall ahfail, then run this to clean up PAM.
+set -euo pipefail
+
+INSTALL_DIR="/usr/local/lib/ahfail"
+BOLD=$(tput bold 2>/dev/null || true)
+RESET=$(tput sgr0 2>/dev/null || true)
+
+step() { echo "${BOLD}==> $*${RESET}"; }
+
+[[ "$(uname)" == "Darwin" ]] || { echo "This script is for macOS only." >&2; exit 1; }
+
+# ── 1. Remove PAM config line ─────────────────────────────────────────────────
+step "Removing ahfail from PAM configuration..."
+removed_pam=0
+for pam_file in /etc/pam.d/screensaverui /etc/pam.d/screensaver; do
+    if [[ -f "$pam_file" ]] && grep -q "ahfail" "$pam_file"; then
+        echo "  Patching ${pam_file}..."
+        sudo sed -i '' '/ahfail/d' "$pam_file"
+        removed_pam=1
+    fi
+done
+[[ $removed_pam -eq 0 ]] && echo "  No ahfail PAM entries found."
+
+# ── 2. Remove binaries ────────────────────────────────────────────────────────
+step "Removing binaries..."
+if [[ -d "$INSTALL_DIR" ]]; then
+    sudo rm -rf "$INSTALL_DIR"
+    echo "  Removed ${INSTALL_DIR}."
+else
+    echo "  ${INSTALL_DIR} not found — skipping."
+fi
+
+echo ""
+echo "${BOLD}Done.${RESET}"