Retire the bridge to the old dev channel release

Scribe #3884. The dev channel's release tag moved from `dev` to `dev-rolling`
on 2026-09-10. Since then, the manifest job has also written latest.json to the
old `dev` release, so that desktop apps installed before the move could update
across. The operator has had two desktop installs and is fine reinstalling,
so the bridge goes. The old release and tag are deleted next, through the forge.

- desktop.yml: the BRIDGE_TAG=dev export is removed.
- write-manifest.sh: the TEMPORARY bridge block is removed.
- ci-requirements.md: the "Transitional" paragraph becomes a note that the tag
  is gone, and that an app installed before 2026-09-10 reinstalls with
  install.sh.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ci-requirements.md

@@ -425,14 +425,9 @@ had fetched it, `git push origin dev` failed with
 tag after a branch; `tests/test_channel_tag.py` and the `update.rs` tests fail if
 one is.
 
-**Transitional, from 2026-09-10:** the old `dev` release still exists so desktop
-apps installed from it can update across — the manifest job writes `latest.json`
-to it too (`BRIDGE_TAG=dev` in `desktop.yml`). Until that release and its tag are
-deleted, fully qualify pushes:
-
-```
-git push origin refs/heads/dev:refs/heads/dev
-```
+The old `dev` release and tag were deleted on 2026-10-08 (Scribe #3884), with the
+bridge that kept writing to them. A desktop app installed before 2026-09-10 reads
+that tag and gets no further updates; reinstall it with `install.sh`.
 
 ## Shell scripts have no CI lane