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

tests/test_learning_round_evidence.py

@@ -373,3 +373,36 @@ def test_round_zero_refuses_fewer_than_three_runs(tmp_path: Path) -> None:
             out_dir=tmp_path / "out",
             k_runs=1,
         )
+
+
+def test_a_round_journals_why_the_proposer_had_nothing_to_say(world) -> None:  # type: ignore[no-untyped-def]
+    """`no_proposal` covers two different worlds: a proposer with nothing to add, and one that could not start.
+
+    Measured: the meta-agent round journalled `no_proposal` while the real cause was an authentication failure in a
+    provider seam pointing at the wrong vendor. A round that failed for a fixable configuration reason must not
+    look identical to a model that considered the facts and declined."""
+
+    class Mute:
+        reason = "meta_agent_failed: RuntimeError"
+
+        def propose(self, facts):  # type: ignore[no-untyped-def]
+            del facts
+            return None
+
+    from openultrasast.learning.rounds import run_learning_round
+
+    record = run_learning_round(
+        world["cases"],
+        family="injection",
+        taxonomy=load_families(),
+        configs_dir=world["configs"],
+        proposer=Mute(),
+        scan_factory=_injection_improves,
+        model="m",
+        journal=world["journal"],
+        archive=world["archive"],
+        floors=world["floors"],
+        out_dir=world["out"],
+    )
+    assert record.outcome == "rejected"
+    assert record.reason == "no_proposal: meta_agent_failed: RuntimeError"