ISO: flip [breadway] to the signed dl.breadway.dev/arch repo

The signed repo is live: https://dl.breadway.dev/arch/x86_64/ serves
breadway.db + .files + every .pkg.tar.zst with a detached .sig from the
BOS release key (56203B86A110695AE7F310934AF3323D678EB5E2 = KEYS.asc),
rebuilt from the Forgejo Arch registry by signed-repo.yml +
scripts/ci-publish-signed-repo.sh. Verified: db/pkg sigs all GPG-good
against KEYS.asc.

Executes the "After the signed repo exists" plan in docs/signed-repo.md:

- `iso/pacman.conf` + `iso/airootfs/etc/pacman.conf`: section renamed
  `[Breadway.os.git.breadway.dev]` → `[breadway]` (matches breadway.db),
  `Server = https://dl.breadway.dev/arch/$arch`, `SigLevel = Required`.
  The old "Forgejo has no db sigs / KEYS.asc is not a repo key / do NOT
  flip" comments are gone — both are now false.
- `iso/airootfs/etc/pacman.d/breadway-repo.asc`: the public key, baked
  into the image.
- `build-local.sh`: trust the key in the build host's pacman keyring
  before mkarchiso (so pacstrap can verify [breadway] while assembling
  the airootfs); drop the now-obsolete Forgejo-registry URL rewrite.
- `iso/airootfs/root/customize_airootfs.sh` (new): trust the key in the
  image keyring so the live medium — and, via calamares unpackfs, the
  installed target — verify [breadway]. (archiso warns this hook is
  deprecated; there is no replacement for "add a repo key to the image
  keyring" and BOS ships no pacman-init.service.)
- `calamares/post-install.sh`: `pacman-key --add` + `--lsign-key` the
  BOS key in the target chroot as a fallback (unpackfs can skip
  /etc/pacman.d/gnupg).
- README.md / DESIGN.md / docs/signed-repo.md updated.

NOT yet done: build the ISO (`sudo ./build-local.sh`) and VM-verify
`pacman -Sy` + a `[breadway]` install with no signature prompt, on both
the live medium and a fresh install. The build-time keyring path
(pacstrap -G vs host keyring vs customize_airootfs) may need a tweak
once the real build runs.

iso/airootfs/root/customize_airootfs.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Run by mkarchiso inside the airootfs chroot, after packages are installed
+# and before the squashfs is built. (archiso prints a deprecation warning for
+# this hook, but there is no non-deprecated replacement for "trust an extra
+# pacman repo key in the image keyring", and BOS ships no pacman-init.service.)
+#
+# Purpose: trust the BOS release key (56203B86…) in the image's pacman
+# keyring so the signed [breadway] repo (SigLevel = Required,
+# https://dl.breadway.dev/arch) verifies both on the live medium and — via
+# calamares' unpackfs, which copies this squashfs to the target — on the
+# installed system. calamares/post-install.sh re-does this in the target
+# chroot as a fallback (unpackfs can skip /etc/pacman.d/gnupg).
+set -euo pipefail
+
+BREADWAY_KEY_FPR="56203B86A110695AE7F310934AF3323D678EB5E2"
+KEY_FILE="/etc/pacman.d/breadway-repo.asc"
+
+pacman-key --init
+pacman-key --populate archlinux
+
+if [[ -f "$KEY_FILE" ]]; then
+    pacman-key --add "$KEY_FILE"
+    pacman-key --lsign-key "$BREADWAY_KEY_FPR"
+    echo "customize_airootfs: trusted [breadway] repo key $BREADWAY_KEY_FPR"
+else
+    echo "customize_airootfs: WARNING $KEY_FILE missing; [breadway] will not verify" >&2
+fi