feat(engine): resolve registry references in sub-workflow workflow: field

[jrob5756]

Closes #172.

What

Extends type: workflow agents to accept registry references (name@registry#ref syntax) in the workflow: field, in addition to local file paths. Connects the existing sub-workflow composition feature (#79) and the workflow registry, enabling cross-team workflow composition by reference.

agents:
  - name: analyze
    type: workflow
    workflow: 'analysis@team-a#v2.0.0'    # NEW — was local path only
    input_mapping:
      repo: "{{ workflow.input.repo }}"

How

Engine (src/conductor/engine/workflow.py)

New _resolve_subworkflow_path() async helper, used by both _execute_subworkflow and _execute_subworkflow_with_inputs. Resolution order:

1. If the value resolves to an existing file relative to the parent workflow directory → return that path. Preserves backward compatibility for bare names like analysis referring to a sibling file without a .yaml extension.
2. Otherwise, parse via registry/resolver.resolve_ref() to detect file vs registry syntax.
3. File-kind that doesn't exist → return the path so the caller emits a clear "file not found" error.
4. Registry-kind → call cache.fetch_workflow() via asyncio.to_thread() (synchronous HTTP) and return the local cached path.

RegistryError from both resolve_ref() and fetch_workflow() is wrapped in ExecutionError with agent context.

Validator (src/conductor/config/validator.py)

conductor validate now resolves and recursively validates sub-workflow references for all type: workflow agents (including for_each inline agents). For registry refs, this fetches the workflow into the local cache during validation. This is a behavioral change: validate now makes network requests when registry refs are present (the issue calls for this explicitly to validate the full composition tree).

Validation is recursive with cycle detection (visited-set threaded through validate_workflow_config via internal kwargs) and a depth cap of 10.

Resume / replay compatibility

The resolver is called on every sub-workflow execution, including after conductor resume. For SHA-pinned refs (name@registry#v1.2.3 or name@registry#<sha>) the cache is content-addressable so resume is fully deterministic. For mutable refs (#main or omitted) resume may pick up a newer commit if the upstream branch has moved — documented in the helper's docstring with a recommendation to pin in production.

Tests

13 new tests covering:

• Engine: registry ref resolves and executes (mocked); registry fetch failure → ExecutionError; extensionless local file takes precedence over registry; malformed ref → ExecutionError; full round-trip resume verifying the same cached path is used on both runs.
• Validator: local path OK; missing local errors; malformed ref errors; registry ref fetch + recursive validate; registry fetch failure errors; for_each inline workflow agent validated; A → B → A cycle detection.

All 2445 tests pass; make check (ruff + ty) clean.

Files

src/conductor/config/validator.py     | +178
src/conductor/engine/workflow.py      | +108
tests/test_config/test_validator.py   | +312 (TestSubWorkflowRefValidation)
tests/test_engine/test_subworkflow.py | +370 (TestRegistrySubWorkflowResolution)

Design note

The registry design doc states "Keep the registry mechanism out of the engine: it is a CLI/loader concern." This PR crosses that line by adding registry resolution to the engine's sub-workflow path-resolution step. Per the issue's analysis, the alternative (pre-fetch and reference by local path) defeats the purpose of composable workflows. The change is contained to a single helper called immediately before load_config().