Add a desktop notification to an agent screenshot script without polluting the shot

When an agent captures the screen with grim, the user gets no feedback (unlike their own screenshot keybind, which usually chains notify-send). Fix: in the capture script, call notify-send -a Screenshot -i "$png" "Agent screenshot" "$png" || true AFTER grim writes the file, so the popup can never appear in that capture. Gate it on an env var (e.g. DESKTOP_SCREENSHOT_NOTIFY=0 to skip) because during back-to-back captures (README screenshot sets) the popup from shot N shows up in shot N+1. Verify by capturing ~0.8s after a notifying capture with the notification disabled: the popup with its thumbnail should be visible in the check shot. NixOS gotcha: if the skill reaches ~/.claude/skills via a home-manager flake input pointing at GitHub, editing the local clone does nothing until you commit, push, run nix flake update <input>, and rebuild.