screenshot: revert flameshot, use grim+slurp+satty pipeline

Flameshot's portal-based capture path on niri triggers an
xdg-desktop-portal access prompt on every invocation, blocking
the screenshot itself. grim talks to wlr-screencopy directly and
never touches the portal, so no prompt.

Pipeline: slurp (region) → grim (capture) → satty (annotate +
copy + save). satty replaces the wofi action menu by being the
post-capture surface itself.

- scripts/screenshot.sh added
- install.sh symlinks it; flameshot.ini generator removed
- niri Mod+Print → "screenshot"
- packages.txt drops flameshot
- docs tables updated

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

niri/config.kdl

@@ -84,7 +84,7 @@ binds {
     Mod+Shift+8 { move-window-to-workspace 8; }
     Mod+Shift+9 { move-window-to-workspace 9; }
 
-    Mod+Print { spawn "flameshot" "gui"; }
+    Mod+Print { spawn "screenshot"; }
 
     Mod+M { move-window-to-workspace "minimized"; }
     Mod+Shift+M { focus-workspace "minimized"; }