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>

ARCHITECTURE.md

@@ -131,11 +131,11 @@ This system is a **Wayland-first desktop environment** built around the **Niri c
 
 ### Background Services
 
-| Function    | Tool         |
-| ----------- | ------------ |
-| Wallpaper   | swww         |
-| Screenshots | flameshot    |
-| Clipboard   | cliphist     |
+| Function    | Tool                 |
+| ----------- | -------------------- |
+| Wallpaper   | swww                 |
+| Screenshots | grim + slurp + satty |
+| Clipboard   | cliphist             |
 
 ---