feat(iso): build end-4 deps in-image, ship end4-pC as default shell

- customize_airootfs.sh stages dots-hyprland at /opt, runs ./setup
  install-deps as a throwaway builder, harvests the built packages into
  /opt/localrepo so pacstrap resolves them offline
- image ships its own [local-aur] (file:///opt/localrepo); CI and
  localrepo/update.sh keep that copy in sync; .gitignore covers work/,
  out/ and airootfs/opt/localrepo/
- alvin-install runs ./setup install --skip-alldeps on the target
- clone pctrade/end4-pC into /etc/skel/.config/quickshell, patch qsConfig
  ii -> end4-pC in the staged variables.lua (guarded + idempotent), and
  restore the fork after setup rsync --delete's ~/.config/quickshell
- ship user presets in skel, point fish `q` alias at end4-pC
- AGENTS.md: document the fourth package source and new gotchas

localrepo/update.sh

@@ -22,11 +22,26 @@ mapfile -t pkgs < <(find "${repo_dir}" -maxdepth 1 -name '*.pkg.tar.zst' -print
 
 if [[ ${#pkgs[@]} -eq 0 ]]; then
     echo "No .pkg.tar.zst files in ${repo_dir}" >&2
+    # keep the medium's copy honest too - an emptied localrepo/ must not
+    # leave the previous packages shipping on the next build
+    rm -rf "${profile_dir}/airootfs/opt/localrepo"
     exit 0
 fi
 
 repo-add -R "${repo_dir}/${repo_name}.db.tar.gz" "${pkgs[@]}"
 
+# Sync the medium's copy: the ISO ships its own [local-aur] at
+# airootfs/opt/localrepo (pacman.conf on the live system points there, and
+# pacstrap resolves these packages from it at install time).
+img_repo="${profile_dir}/airootfs/opt/localrepo"
+mkdir -p "${img_repo}"
+rm -f "${img_repo}"/*.pkg.tar.zst "${img_repo}/${repo_name}".db \
+    "${img_repo}/${repo_name}".db.tar.gz "${img_repo}/${repo_name}".files \
+    "${img_repo}/${repo_name}".files.tar.gz
+cp -f "${pkgs[@]}" "${img_repo}/"
+cp -fL "${repo_dir}/${repo_name}".db* "${repo_dir}/${repo_name}".files* "${img_repo}/"
+echo "Medium copy ready: ${img_repo}"
+
 new_names=()
 for p in "${pkgs[@]}"; do
     name=$(tar -xOf "${p}" .PKGINFO | sed -n 's/^pkgname = //p' | head -1)