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/etc/pacman.d/breadway-repo.asc

@@ -0,0 +1,15 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+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+=Mrh1
+-----END PGP PUBLIC KEY BLOCK-----