fix(learning-harness): say why the proposer had nothing, and stop hard-coding its vendor

The meta-agent round journalled `no_proposal`. The real cause was an
authentication failure: `_model_config` hard-coded the anthropic provider
while the detector was talking to DeepSeek and no ANTHROPIC_API_KEY exists
here. The composition itself was fine -- MetaAgent constructed, evolve
started, budget $5 -- and the trace was thrown away.

`no_proposal` covered two different worlds: a proposer that considered the
facts and declined, and one that could not start at all. The journal now
carries the proposer's own reason, and the vendor comes from
`[harnessx] provider` instead of being assumed.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

src/openultrasast/learning/rounds.py

@@ -418,7 +418,18 @@ def run_learning_round(
     proposal = proposer.propose(facts)  # type: ignore[attr-defined]
     record = _round_record(round_number, family, proposal, taxonomy, config)
     if proposal is None:
-        return _finish(journal, directory, record, outcome="rejected", reason="no_proposal", cost=spent, trajectories=trajectories)
+        # `no_proposal` covers two different worlds: a proposer that considered the facts and declined, and one
+        # that could not start at all. Losing that distinction hides a fixable configuration error as a quiet round.
+        why = str(getattr(proposer, "reason", "") or "")
+        return _finish(
+            journal,
+            directory,
+            record,
+            outcome="rejected",
+            reason=f"no_proposal: {why}" if why else "no_proposal",
+            cost=spent,
+            trajectories=trajectories,
+        )
     refusal = refuse_proposal(proposal, config)
     if refusal is not None:
         return _finish(