sync: ride the resolved remote, and say when there is none

REMOTE/UPSTREAM were module constants, so a team-mode board whose remote
is named anything but origin synced nothing at all — silently, with a
healthy header. The remote is now resolved per use (config's answer, the
one PRs already honour) and threaded through fetch, publish, replay and
integrate; every message names whatever was resolved.

The silence is the other half. Where _converge()/push_now() returned a
bare 'no-origin' that both callers discarded, _remote() now _note()s at
stalled level first: no remote at all names both fixes (add one, or set
BOARD_GIT_REMOTE), and a BOARD_GIT_REMOTE naming a remote this checkout
does not have stalls naming it rather than quietly using another. It
clears with a recovery line like the offline path, and install() asks at
startup so the chip is there from first paint, not the second beat.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -635,6 +635,17 @@ no push, no thread, no behaviour change at all.
 - **Offline is quiet.** An unreachable origin says so once, then works
   locally; commits queue on `main` and go out on the next reachable
   fetch.
+- **Which remote is one answer, not two.** One remote and one branch, by
+  design — but the remote is the one `BOARD_GIT_REMOTE` names, else the
+  checkout's first, resolved in the same place PR opening asks, so the two
+  halves of team mode can never publish to different places. It is used as
+  named: a `BOARD_GIT_REMOTE` this checkout has no remote for stalls
+  saying so rather than reaching past it for another one. A checkout with
+  no remote at all stalls the same way, from startup — team mode syncing
+  nothing is exactly the state a board must not render as healthy, and it
+  is the likeliest first state of a fresh installation. Add a remote (or
+  set the setting) and the chip clears with a line saying sync is
+  converging again.
 
 ### State syncs; reactions don't