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_obligation_export.py

@@ -133,7 +133,7 @@ def test_the_train_split_of_vibe_py_teaches_no_obligation_shape_today(tmp_path:
     assert refused["threatbyte-api-v1-get"] == "split:holdout"
     # Its own reason, not its split: this row is a byte-identical twin that happens to sit on the holdout side,
     # and reporting `split:holdout` would hide a corpus fact behind a split fact (Req 5.2).
-    assert refused["vampi-books-get-by-title"] == "unscorable:identical_twin"
+    assert refused["vampi-books-get-by-title"] == "identical_twin"
     flow_rows = [row for row in store.load() if row.shape is not None and row.shape.get("family", "sink") == "sink"]
     assert report.records == len(flow_rows) and len(flow_rows) >= 1  # the flow family still teaches from the train split