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

.gitignore

@@ -0,0 +1,7 @@
+# Multi-GB mkarchiso output (see AGENTS.md build command)
+work/
+out/
+
+# In-image copy of localrepo/ — populated by localrepo/update.sh and
+# .gitea/workflows/build-iso.yaml, shipped as /opt/localrepo on the ISO
+airootfs/opt/localrepo/