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

.gitea/workflows/build-iso.yaml

@@ -22,9 +22,21 @@ jobs:
       - name: Setup local AUR repo
         run: |
           set -euo pipefail
+          shopt -s nullglob
           mkdir -p /tmp/localrepo
           cp localrepo/*.pkg.tar.zst localrepo/local-aur.db* localrepo/local-aur.files* /tmp/localrepo/
           sed -i 's|file:///home/alvin/AlvinArch/localrepo|file:///tmp/localrepo|' pacman.conf
+          # The medium needs its own copy: the ISO's /etc/pacman.conf points
+          # [local-aur] at file:///opt/localrepo, which pacstrap reads at
+          # install time to resolve packages that are not in the official repos
+          # (built during the ISO build and harvested there by
+          # airootfs/root/customize_airootfs.sh).
+          mkdir -p airootfs/opt/localrepo
+          shipped=(localrepo/*.pkg.tar.zst localrepo/local-aur.db* localrepo/local-aur.files*)
+          if (( ${#shipped[@]} > 0 )); then
+            cp -L "${shipped[@]}" airootfs/opt/localrepo/
+          fi
+          echo "airootfs/opt/localrepo: $(ls airootfs/opt/localrepo | wc -l) file(s)"
 
       - name: Install archiso and dependencies
         run: |