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.conf

@@ -32,18 +32,17 @@ Include = /etc/pacman.d/mirrorlist
 # are NOT here; they are bakery-baked into /usr/local at ISO build time.
 #
 # Packages are published to the Forgejo Arch registry (group "os") by the
-# .forgejo/workflows/*.yml workflows in this repo (and breadlock's).
+# .forgejo/workflows/*.yml workflows; scripts/ci-publish-signed-repo.sh then
+# collects them, detach-signs each .pkg.tar.zst with the BOS release key
+# (releases@breadway.dev), runs `repo-add -s`, and publishes the signed db
+# at https://dl.breadway.dev/arch/$arch (signed-repo.yml).
 #
-# Forgejo's Arch package registry does not serve pacman-compatible db
-# signatures. SigLevel = Never is TLS-only integrity: the connection is
-# HTTPS (or rewritten to hestia's localhost:3002 in CI). breadlock (PAM)
-# rides this repo. Do NOT flip to SigLevel = Required unless a signed db
-# has been verified to work — Required without signatures breaks the ISO
-# and every install that uses [breadway]. KEYS.asc is the ISO SHA256SUMS
-# signing key, not a pacman repo key.
+# SigLevel = Required: every package AND the db carry a .sig from key
+# 56203B86A110695AE7F310934AF3323D678EB5E2 — the same key committed as
+# KEYS.asc / etc/pacman.d/breadway-repo.asc, imported into the pacman
+# keyring at build time (build-local.sh), on the live medium, and on the
+# installed target (calamares/post-install.sh).
 # -----------------------------------------------------------------------
-# The section name must match Forgejo's served db filename
-# ({owner}.{group}.{domain}.db) — pacman fetches "<section>.db" from Server.
-[Breadway.os.git.breadway.dev]
-SigLevel = Never
-Server = https://git.breadway.dev/api/packages/Breadway/arch/os/$arch
+[breadway]
+SigLevel = Required
+Server = https://dl.breadway.dev/arch/$arch