Wraps devagentic's ``assertOutput(callId, expectation:
{fragment, predicate?}) -> Verdict`` mutation as an MCP tool.
Workers can now vet a prior tool_call's output against a
predicate without shelling out to ``python api.py``.

## Implementation

- ``plugins/devagentic-mutations/client.py::assert_output(call_id,
  fragment, predicate=None)`` — thin GraphQL wrapper following the
  ``query_silo`` / ``run_confer_loop`` pattern. Returns the verdict
  dict ``{id, ts, callId, passed, violations}`` on success or
  ``None`` on failure (auth, network, ``errors`` payload).

- ``mcp_serve.py::_register_devagentic_mutation_tools`` — adds the
  ``assert_output`` MCP tool. Flat args ``(call_id, fragment,
  predicate=None)``; mapped to the ``ExpectationInput`` GraphQL
  shape internally. Failure path returns ``{"error": "..."}`` JSON
  with the plugin's ``last_error_text()`` surface threaded in.

## Predicate stiffener interop

When ``predicate`` is omitted or empty, the wrapper passes ``None``
to devagentic — explicitly matching the resolver's
``_stiffen_predicate(tool, None) -> tool-specific floor`` path.
This is what triggers the ``kind:predicate-coerced`` telemetry doc
on the devagentic side (arc-6 audit trail). Tested with two
parametric cases: omitted predicate, empty-string predicate.

## Tests

- 7 new client-level tests in tests/plugins/devagentic-mutations/
  test_client.py: empty-arg fail-soft, missing user_id fail-soft,
  happy-path verdict round-trip, predicate-omitted-sends-null,
  empty-predicate-sends-null, null-data fail-soft, non-dict-response
  fail-soft.
- Contract test in tests/test_mcp_defensive_registrars.py extended:
  ``assert_output`` is now in the required-G2-tools set, so a future
  regression that drops the registrar will surface in CI.
- 182 total green across the affected suites (mutations + MCP +
  canvas + docs + subset-filter + autowire). No regression.

## Tracking

- Closes hermes-agent#60 (G2b of #56).
- G2c (#61: patchArtifact / readArtifact) and G2d (#62: fetchUrl)
  remain open as sub-issues.