feat(keybinds): capture region/screen to file

Add SHIFT+Print (region → file) and CTRL+Print (screen → file) next to
the existing bare Print (region → clipboard). Each saves a timestamped
PNG under ~/Pictures/Screenshots and toasts the path — the same grim/slurp
plumbing and directory the Capture menu's "→ file" rows already use.
Single-source keybinds.nix, so the SUPER+? cheatsheet lists both new rows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -263,13 +263,6 @@ mime defaults (item 8's `xdg.mimeApps` surface) so audio/video files open
 in them. (mpv already the video default per HARDWARE-QUEUE — confirm and
 extend to audio; pick the audio player.)
 
-### 38. Capture-to-file keybinds
-(Raised by Bernardo, 2026-07-07 — item 8.) Add binds for "capture region
-→ file" and "capture screen → file" (distinct from the existing
-clipboard/recording capture flows), landing PNGs in ~/Pictures. Add to
-`keybinds.nix` (single source → Hyprland + the SUPER+? cheatsheet) and
-the Capture menu; reuse the existing grim/slurp plumbing.
-
 ### 41. Floating-window audit
 (Raised by Bernardo, 2026-07-07 — item 11.) Decide which windows should
 open floating by default (config dialogs, pavucontrol, file-chooser