Wraps devagentic's ``executeWorkflowPipeline`` (NousResearch#258) +
``writeWorkflowPipeline`` (NousResearch#261) mutations as MCP tools so workers
can compose + execute pipelines directly through their tool
surface. R12 of devagentic#210 (flow-aware router umbrella).

## Tools registered

| Tool | Wraps | Returns |
|---|---|---|
| ``run_pipeline(pipeline_id, user_id)`` | ``executeWorkflowPipeline`` | ``PipelineRun {runId, status, nodeOutcomes, ...}`` |
| ``propose_pipeline(name, intent, version, nodes, edges, user_id, lineage_ref?, produced_by?)`` | ``writeWorkflowPipeline`` | ``Doc {id, content, tags, source, ts}`` |

Composes naturally: ``propose_pipeline → doc_id → run_pipeline``.

## Devagentic-side resolver verification

Schemas read directly from devagentic ``api.py`` via gh api:

- ``executeWorkflowPipeline(pipelineId: String!, userId: String!) -> PipelineRun`` (api.py:21628)
- ``writeWorkflowPipeline(name, intent, version, nodes, edges, userId, lineageRef?, producedBy?) -> Doc`` (api.py:21576)

Arg-name camelCase conversion (``pipeline_id`` → ``pipelineId``,
``lineage_ref`` → ``lineageRef``, etc.) matches strawberry's default.

Note: #100's body referenced ``ShadowNodeRef`` as the propose_pipeline
return — the actual mutation returns ``Doc``. Implementing the real
shape.

## Tests

- 10 new client-level tests in
  ``tests/plugins/devagentic-mutations/test_client.py``:
  * ``run_pipeline``: empty-args fail-soft (1), happy path with arg
    threading + nested nodeOutcomes (1), non-dict fail-soft (1).
  * ``propose_pipeline``: empty-args fail-soft (3),
    invalid-version-type fail-soft (3 parametrized), invalid
    nodes/edges-type fail-soft (2), happy path with all args
    threaded (1), optional-args-null-when-omitted (1), non-dict
    fail-soft (1).
- Contract test in
  ``tests/test_mcp_defensive_registrars.py`` extended:
  ``run_pipeline`` + ``propose_pipeline`` added to the required-G2/R12
  tool set (now 17 named devagentic tools).
- 184 total green across affected suites — no regression.

## Composition

Unblocks the worker-side pipeline-economy use case in
devagentic#210 R12. Workers can now author + execute pipelines
without shelling out to ``python api.py``. The recent #99 / #110
fixes (PRs #108 + #111) ensure tool_calls with empty content +
populated tool_calls on Groq/strict-validator paths don't cascade
through the empty-retry loop, so R12 tools are usable from
client-tier dispatches.