fix(learning-harness): the round-4 review findings

Three defects, two of them the same shape as ones already rejected once:

- The meta-agent proposer still could not propose. `MetaAgent(inner_model=)`
  takes a ModelConfig; it was handed a model name, a `# type: ignore` hid
  that from mypy, and a bare `except Exception` turned the resulting
  AttributeError into `meta_agent_failed` -- which reads exactly like the
  model declining. The composition is now a seam a test can pin, and a
  construction error is re-raised rather than reported as a quiet round.
- `ousast pairs --hunter` crashed in its documented form: the K floor landed
  with `--k-runs` still defaulting to 1. The covering test always passed
  `--k-runs 4`, which is why nothing noticed.
- 28 rows (12% of the corpus) resolved a span on the vulnerable side and
  none on the fixed side, so every fixed-side finding read as outside the
  labeled function and the pair scored as silence whatever the detector
  said. `fix_function` now reaches the catalog and the scorer, and the
  generator derives it from the fixed excerpt for the 25 trap rows that
  never declared one. Measured after: 0 rows.

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

tests/test_loo.py

@@ -89,7 +89,9 @@ def test_leave_one_out_seeds_only_trusted_pairs_and_skips_known_limit(tmp_path:
     assert set(by_name) == {"a", "b"}  # known_limit excluded entirely
     assert not by_name["a"].detected  # b is advisory: it cannot teach a
     assert by_name["b"].detected  # a (seeded) teaches b, which is scored as a held-out target
-    assert result.skipped == (("c", "known_limit:engine"),)
+    # `b` is excluded from teaching for its tier and says so; without that line a pair that silently stopped
+    # teaching would look identical to one that never could (Req 5.2).
+    assert result.skipped == (("c", "known_limit:engine"), ("b", "not_a_teacher:tier"))
 
 
 def test_cli_pairs_loo_writes_the_artifact_offline(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None: