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_publish.py

@@ -187,8 +187,13 @@ def test_two_slices_of_one_model_are_two_rows_not_one_overwriting_the_other(tmp_
                     "taxonomy_version": "1",
                     "metrics": {
                         "injection": {
-                            "family": "injection", "scorable": 10, "unscorable": {}, "recall": recall,
-                            "silence": 1.0, "youden": recall, "hierarchical_credit": False,
+                            "family": "injection",
+                            "scorable": 10,
+                            "unscorable": {},
+                            "recall": recall,
+                            "silence": 1.0,
+                            "youden": recall,
+                            "hierarchical_credit": False,
                             "outcomes": {"pair_correct": 5},
                         }
                     },