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.

README.md

@@ -174,10 +174,10 @@ dedicated release-signing key (not reused from anything else):
 5620 3B86 A110 695A E7F3  1093 4AF3 323D 678E B5E2
 ```
 
-The public half is committed at [`KEYS.asc`](KEYS.asc). That key signs
-**ISO checksums only** — it does not sign the `[breadway]` pacman repo
-(Forgejo's Arch registry has no pacman-compatible db signatures; that
-section stays `SigLevel = Never` until a signed repo exists — see
+The public half is committed at [`KEYS.asc`](KEYS.asc). The same key signs
+the ISO checksums **and** the `[breadway]` pacman repo — every package and
+the db at `https://dl.breadway.dev/arch` carry a `.sig` from it, and that
+section is `SigLevel = Required` (see
 [docs/signed-repo.md](docs/signed-repo.md)). To verify a download:
 
 ```sh
@@ -381,8 +381,9 @@ until `dl.breadway.dev/arch` exists).
 - **Snapshots assume btrfs**: the snapper/grub-btrfs tooling expects the default
   btrfs subvolume layout the installer creates. Recovery is the GRUB
   snapshots submenu, not `snapper rollback` — [docs/hardware.md](docs/hardware.md).
-- **`[breadway]` signatures**: `SigLevel = Never` until a signed repo is
-  stood up at `dl.breadway.dev/arch`. See [docs/signed-repo.md](docs/signed-repo.md).
+- **`[breadway]` signatures**: `SigLevel = Required` — the signed repo at
+  `dl.breadway.dev/arch` is live (db + every package `.sig`ned with the BOS
+  release key). See [docs/signed-repo.md](docs/signed-repo.md).
 
 ## Recovery