fix(ci): ensure /dev/stdin exists for mkinitcpio and remove stock linux kernel

- Add CI step to create /dev/stdin symlinks before ISO build
  (Docker/container environments may lack these, causing mkinitcpio to fail)
- Remove stock linux kernel from packages.x86_64 (linux-cachyos is the
  intended kernel; dual-kernel causes boot confusion and doubles build time)

Closes #X

.gitea/workflows/build-iso.yaml

@@ -31,6 +31,14 @@ jobs:
           set -euo pipefail
           pacman -Syu --noconfirm --needed archiso jq git-lfs zstd
 
+      - name: Ensure /dev/stdin exists for mkinitcpio
+        run: |
+          set -euo pipefail
+          # Docker/container environments may lack /dev/stdin; mkinitcpio needs it
+          [ -e /dev/stdin ]  || ln -sf /proc/self/fd/0 /dev/stdin
+          [ -e /dev/stdout ] || ln -sf /proc/self/fd/1 /dev/stdout
+          [ -e /dev/stderr ] || ln -sf /proc/self/fd/2 /dev/stderr
+
       - name: Build Arch ISO
         run: |
           set -euo pipefail