fix(window-restore): re-read workspace/window state after the wofi pick

The picker blocks on user input for however long they take. In that
window niri can destroy a dynamic workspace (its last window closed)
and renumber every later one down, so the pre-pick current["idx"] can
point at the wrong workspace by the time move-window-to-workspace
runs — restoring to the wrong place. The stashed window itself can
also have closed in the meantime, in which case the move/focus calls
would silently no-op. main() now re-reads workspaces/windows after
pick() returns, recomputes the current workspace, and confirms the
chosen window still exists before acting; each bails with a notify
and the documented exit code otherwise. Adds window_exists() as a
pure, tested helper alongside the existing pure functions.

Also: clarify the focus=false comment in niri/config.kdl (it read as
if focus=false caused the problem it actually prevents), and note in
the design doc's Out of scope section that the stash's index-1
pinning is per-output, so a second monitor would need its own
analysis of the Mod+1..9 offset.

niri/config.kdl

@@ -110,8 +110,8 @@ binds {
 
     Mod+Print { spawn "screenshot"; }
 
-    // focus=false or focus follows the window into the stash — the opposite of
-    // minimizing.
+    // focus=false is required here — otherwise focus follows the window into
+    // the stash, the opposite of minimizing.
     Mod+M { move-window-to-workspace "minimized" focus=false; }
     Mod+Shift+M { spawn "window-restore"; }