chore(release): archive ISO build recipes, tarball-only releases

Releases no longer ship as bootable ISOs. Archipelago updates are
distributed as the backend binary plus a frontend tarball referenced by
releases/manifest.json. Nodes OTA-update via scripts/self-update.sh.

Filebrowser and AIUI remain bundled inside the frontend tarball and
deployed atomically, verified present in v1.7.43-alpha release artifact
(189 AIUI files, filebrowser-client bundle).

Archived under image-recipe/_archived/ (resurrectable if ISO distribution
is reintroduced):
  - build-auto-installer-iso.sh
  - build-unbundled-iso.sh
  - test-iso-qemu.sh
  - scripts/convert-iso-to-disk.sh
  - BUILD-ISO-STATUS.md, ISO-BUILD-CHECKLIST.md
  - branding/isohdpfx.bin
  - .gitea/workflows/build-iso-dev.yml

Updated release process docs to drop ISO references:
  - scripts/create-release.sh (next-steps text)
  - docs/BETA-RELEASE-CHECKLIST.md
  - docs/hotfix-process.md
  - README.md

image-recipe/_archived/README.md

@@ -0,0 +1,37 @@
+# Archived ISO build recipes
+
+These scripts built the Archipelago auto-installer ISO (bundled and
+unbundled variants). As of v1.7.43-alpha, ISOs are no longer part of the
+release deliverable. Releases ship as tarballs consumed by
+`scripts/self-update.sh` on existing nodes.
+
+Archived here rather than deleted so they can be resurrected if ISO
+distribution is reintroduced.
+
+## Contents
+
+- `build-auto-installer-iso.sh` — orchestrator, bundles container images into squashfs
+- `build-unbundled-iso.sh` — thin wrapper that sets BUNDLE_IMAGES=0 and delegates
+- `test-iso-qemu.sh` — smoke-tests a built ISO under QEMU
+- `scripts/convert-iso-to-disk.sh` — converts an ISO to a raw disk image
+- `BUILD-ISO-STATUS.md`, `ISO-BUILD-CHECKLIST.md` — contributor guides
+- `branding/isohdpfx.bin` — isolinux MBR hybrid image
+- `.gitea-workflows/build-iso-dev.yml` — CI workflow that ran the build+smoke-test
+
+## To resurrect
+
+1. `git mv image-recipe/_archived/* image-recipe/` (adjust paths back)
+2. Restore `.gitea/workflows/build-iso-dev.yml`
+3. Re-add release-process references (see `scripts/create-release.sh`,
+   `docs/BETA-RELEASE-CHECKLIST.md`, `docs/hotfix-process.md`, `README.md`).
+
+## Why archived
+
+The release flow is simpler and faster as tarball-only:
+- `releases/vX.Y.Z-alpha/archipelago` (backend binary)
+- `releases/vX.Y.Z-alpha/archipelago-frontend-X.Y.Z-alpha.tar.gz` (frontend + AIUI + filebrowser UI assets)
+- `releases/manifest.json` (pointers + changelog)
+
+Nodes pull these via `scripts/self-update.sh` from either Gitea mirror.
+Filebrowser and AIUI remain bundled inside the frontend tarball and deployed
+atomically by `self-update.sh`.