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_learning_refusals.py

@@ -45,7 +45,7 @@ def test_a_train_split_twin_is_refused_for_being_a_twin_not_for_its_split(tmp_pa
     `vulnpy-deserialization-do-pickle-load` are train-split identical twins."""
     from openultrasast.semantic.seed import _skip_reason
 
-    assert _skip_reason(_case(tmp_path, "twin", split="train", unscorable="identical_twin")) == "unscorable:identical_twin"
+    assert _skip_reason(_case(tmp_path, "twin", split="train", unscorable="identical_twin")) == "identical_twin"
     assert _skip_reason(_case(tmp_path, "held", split="holdout")) == "split:holdout"
     assert _skip_reason(_case(tmp_path, "good", split="train")) is None