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_cutoff.py

@@ -44,6 +44,9 @@ def test_every_vendored_row_carries_the_date_its_recipe_recorded() -> None:
         ("2026", "2026-03-01", False),  # a year-precision row that might be before it is not claimed
         ("2026-04-02", "2026-03-01", True),
         ("2024", "2025-07-01", False),
+        ("2026-05", "2026-03-01", True),  # month precision: the earliest day of May is after 1 March
+        ("2026-05", "2026-06-01", False),  # and before 1 June, so the row is not claimed
+        ("2025", "2025", False),  # a cutoff coarser than a day is refused rather than compared
         ("", "2025-07-01", False),
     ],
 )