feat(devshell): add terminal UI capture tools (#69629)

nix/devShell.nix

@@ -41,6 +41,11 @@
           # libglvnd provides libEGL.so.1 that cage needs on NixOS.
           cage
           libglvnd
+          # Graphical terminal + Wayland screenshot client for CLI/TUI UI
+          # evidence. `cage -- ghostty ...` keeps captures off the user's
+          # live compositor; grim runs inside that isolated client session.
+          ghostty
+          grim
         ]
         ++ self'.packages.default.passthru.devDeps;
         shellHook = ''

scripts/capture-cage-terminal.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+out=${1:?usage: $0 OUTPUT.png [command...]}
+shift
+mkdir -p "$(dirname "$out")"
+
+# This runs inside `cage --`, after Ghostty has already opened the isolated
+# Wayland display. Capture before this script exits so Cage keeps the surface
+# alive long enough for grim to see it.
+if (($#)); then
+  "$@"
+fi
+if ! command -v grim >/dev/null 2>&1; then
+  echo "grim is required inside the Cage client session" >&2
+  exit 127
+fi
+grim "$out"