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_cli.py

@@ -161,7 +161,9 @@ def test_a_round_runs_from_the_command_line_and_journals_its_outcome(workspace:
     assert len(rounds) == 1 and rounds[0]["family"] == "injection"
     # Without the extra there is no meta-agent, so the honest outcome is exactly this one. Accepting any member of
     # the enum would have hidden that the round could never propose anything at all.
-    assert (rounds[0]["outcome"], rounds[0]["reason"]) == ("rejected", "no_proposal")
+    # The reason now carries *why* the proposer had nothing: "no proposal" and "could not start" are different
+    # worlds, and only one of them is a fixable configuration error.
+    assert rounds[0]["outcome"] == "rejected" and rounds[0]["reason"].startswith("no_proposal")
     assert (workspace / "learning" / "rounds" / "1" / "attribution.json").is_file()