build: install catcrafts-fp6-repo so images subscribe to the apk registry

The catcrafts-fp6-repo aport (signing key + repositories entry) existed
but nothing installed it: images shipped with the CI-built packages
baked in and no update channel, so 'apk upgrade' never delivered new
kernel/imsd builds despite CI publishing every apk to the registry.
Add it to extra_packages and document the registry in the README.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -83,3 +83,8 @@ with the `aports/` overlay in this repository:
 - `main/postmarketos-config-nftables` — adds the `-imsd` subpackage accepting
   imsd's IPsec-protected SIP ports on the IMS PDN (auto-installed alongside
   imsd).
+- `device/catcrafts-fp6-repo` — subscribes the installed system to the
+  [Catcrafts apk registry](https://forgejo.catcrafts.net/Catcrafts/-/packages)
+  (signing key + `/etc/apk/repositories` entry). CI publishes every built
+  package there, so `apk upgrade` delivers kernel/imsd/GNSS updates between
+  image releases.

build.sh

@@ -152,7 +152,7 @@ aports = $WORK/pmaports
 device = fairphone-fp6
 ui = plasma-mobile
 systemd = always
-extra_packages = soc-fairphone-fp6-audio,callaudioshim,imsd,fp6-device-tweaks,postmarketos-base-ui-audio-backend-pipewire,pipewire-pulse,pipewire-echo-cancel
+extra_packages = soc-fairphone-fp6-audio,callaudioshim,imsd,fp6-device-tweaks,catcrafts-fp6-repo,postmarketos-base-ui-audio-backend-pipewire,pipewire-pulse,pipewire-echo-cancel
 EOF
 
 pmbootstrap checksum linux-postmarketos-qcom-milos