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/calamares/post-install.sh

@@ -67,15 +67,23 @@ passwd -l root || true
 # over to the target (unpackfs may skip it / perms differ), leaving the installed
 # system unable to verify package signatures — the first `pacman -Syu` then dies
 # with "keyring is not writable / required key missing". Initialise it here so a
-# fresh install can update out of the box. archlinux-keyring is already present
-# and is the only keyring populated — it verifies official Arch packages.
-# [breadway] stays SigLevel=Never (Forgejo does not serve pacman-compatible
-# db signatures). Do not import KEYS.asc here: that key signs ISO SHA256SUMS,
-# not the pacman repo; treating it as a repo key would be a lie.
+# fresh install can update out of the box. archlinux-keyring verifies official
+# Arch packages; the BOS release key (56203B86…, shipped at
+# /etc/pacman.d/breadway-repo.asc) verifies the signed [breadway] repo at
+# dl.breadway.dev/arch — SigLevel = Required there, every package and the db
+# carry a .sig from it.
 # ---------------------------------------------------------------------------
+BREADWAY_KEY_FPR="56203B86A110695AE7F310934AF3323D678EB5E2"
 if command -v pacman-key &>/dev/null; then
     pacman-key --init || echo "WARN: pacman-key --init failed"
     pacman-key --populate archlinux || echo "WARN: pacman-key --populate failed"
+    if [[ -f /etc/pacman.d/breadway-repo.asc ]]; then
+        pacman-key --add /etc/pacman.d/breadway-repo.asc \
+            && pacman-key --lsign-key "$BREADWAY_KEY_FPR" \
+            || echo "WARN: could not trust the [breadway] repo key — pacman -Sy will fail on [breadway]"
+    else
+        echo "WARN: /etc/pacman.d/breadway-repo.asc missing — [breadway] (SigLevel=Required) will not verify"
+    fi
 fi
 
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