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/JOURNAL.md

@@ -17,6 +17,19 @@ Template:
 
 ---
 
+## 2026-07-07 — Capture-to-file keybinds (iteration #59, item 38)
+- **Task:** BACKLOG item 38 — direct keybinds for region→file and
+  screen→file (the Capture *menu* already had both "→ file" rows).
+- **Fix:** `modules/home/keybinds.nix` — added `SHIFT+Print` (region→file)
+  and `CTRL+Print` (screen→file) alongside the existing bare `Print`
+  (region→clipboard). Each saves a timestamped PNG under
+  ~/Pictures/Screenshots and toasts the path — same grim/slurp plumbing +
+  dir the menu's "→ file" rows use. Single-source, so the SUPER+?
+  cheatsheet (rofi.nix reads the same list) gets both rows automatically.
+- **Verified:** V0 green. `nix eval` of the rendered binds confirms
+  `$HOME`/`$f`/`$(...)` pass through literally to the shell (no Nix
+  interpolation). Actual key-firing + file landing is a V3 check (queued).
+
 ## 2026-07-07 — rofi search mis-ranks + power-profile icons (iteration #58, items 31 & 39)
 - **#31 (rofi search broken):** Bernardo ran the live A/B — `-sorting-method
   normal` and `-matching normal` both fixed "steam"→Steam; `-no-sort` did