channels: the dev channel publishes on dev-rolling, so its tag stops shadowing the branch

The rolling dev release lived on a tag named `dev`, beside the branch
named `dev`. Once a clone had fetched tags, `git push origin dev` failed
with "src refspec dev matches more than one" (Scribe #2184, note #3042),
and every session had to know to spell out refs/heads/dev.

The channel is still `dev` everywhere a person sees it: the app's
setting, `install.sh --channel dev`, the stored pref. Only the release
tag moves, to `dev-rolling`, matching roundtable-android. `stable` has no
branch to collide with and keeps its name.

- packaging/channel-tag.sh is the one channel -> tag mapping CI reads:
  the publish steps in android.yml and desktop.yml, the manifest job,
  fetch-clients.sh and guard-forward.sh. guard-forward exits 2 on an
  unmapped channel instead of fetching an empty URL and passing.
- update.rs and install.sh carry their own copy because neither can run
  it; update.rs gains a test that no channel feed is named like a branch.
- tests/test_channel_tag.py runs the script: no tag is a branch name,
  dev is exactly dev-rolling, an unknown channel fails with no output.
- publish-release.sh titles the release "ThoughtSync dev (rolling)", so
  the tag name does not leak into what people read.

TEMPORARY bridge: desktop apps installed before this have
.../download/dev/latest.json compiled in. The dev manifest job sets
BRIDGE_TAG=dev, and write-manifest.sh writes the same latest.json to
the old `dev` release. Its URLs name dev-rolling assets, so those apps
update once into a build that reads the new tag. The bridge, and the old
release and tag, are removed once installed apps have crossed over.
Until then the push still needs the explicit refspec, as
ci-requirements.md now says.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DwoKYuw3qJmUUYsJeNherB

desktop/packaging/install.sh

@@ -93,7 +93,16 @@ say "Finding the latest ThoughtSync build on the $channel channel…"
 # are pruned to the current build, so the tag alone names the newest build on that
 # channel — which is exactly what an installer wants and what the in-app updater
 # already reads.
-json="$(curl -fsSL "$API/releases/tags/$channel" 2>/dev/null)" ||
+# The channel's RELEASE TAG, which is not always its name: `dev` publishes on the
+# tag `dev-rolling`, because a tag called `dev` shadowed the branch of the same name
+# and broke `git push origin dev` (Scribe #2184). Same mapping as
+# packaging/channel-tag.sh — inlined because this script is fetched alone through a
+# pipe and has nothing to source.
+case "$channel" in
+  dev) tag=dev-rolling ;;
+  *)   tag="$channel" ;;
+esac
+json="$(curl -fsSL "$API/releases/tags/$tag" 2>/dev/null)" ||
   die "the $channel channel has nothing published yet."
 
 # Pull asset URLs straight out of the release JSON (no jq). Anchored on the closing