The branch was named as a one-off dated merge (merge/upstream-dev-20260719),
but it is actually the standing development line: upstream merges land here and
the dev/beta firmware channel is built from it. Rename accordingly and reframe
the handoff so future upstream merges land ON this branch rather than spawning a
new dated branch each time.
Adds a "Branch and Release Channels" section with the full production vs
dev/beta separation table (branch, workflow, release tag, manifest base,
download host, flasher config, embedded version), and restates why both channels
share FIRMWARE_VERSION: the OTA logic treats a differing base as "always an
update", so channels must separate by manifest URL, never by base version.
Also corrects the beta workflow's dispatch-only rationale, which cited a
short-lived branch name that no longer applies. Dispatch-only still stands, for
the better reason: publishing firmware that real nodes pull over the air should
be explicit, not a side effect of every commit to a dev branch.