feat(deepagent): datastores= parity for create_research_workflow

[fede-kamel]

Follow-up to #203 / v0.2.0b11. Closes the only remaining surface gap between create_deepagent and create_research_workflow: the workflow factory now accepts the same datastores= mapping.

Why

create_deepagent(datastores={...}) shipped in v0.2.0b11 with auto-wired search_<name> tools + a per-store routing block prepended to the system prompt. create_research_workflow — the StateGraph variant with the post-hoc grounding loop — exposed only tools=, so users had to hand-wire the retrieval tools via locus.rag.tools.create_rag_tool and write the routing block themselves.

After this change, the two factories are surface-identical for the datastore path:

# Same `datastores=` shape works for both
workflow = create_research_workflow(
    model=...,
    tools=[],
    output_schema=Report,
    datastores={"medical": {"retriever": medical_retriever, "top_k": 6}},
    grounding_threshold=0.65,
)

Implementation

• factory.py: extracted the auto-wiring logic into a new public wire_datastores(datastores, datastore_top_k) -> (tools, routing_block) helper. create_deepagent now calls it (semantics unchanged from b11 — same tools, same prompt block, same TypeError on bad values).
• workflow.py: added datastores= + datastore_top_k= kwargs to create_research_workflow; the execute node sees tools + ds_tools and the prepended routing block.
• docs/concepts/deepagent.md: workflow datastores= example + cross-ref under "When to use each".
• tests/unit/test_research_workflow_datastores.py: 8 cases covering None/empty input, bare-retriever vs dict form, multi-store routing, TypeError on bad value types, and workflow construction with a configured datastore.

Test plan

• 8 new unit tests pass locally
• wire_datastores smoke-tested in isolation
• create_deepagent semantics unchanged (regression via existing tests + smoke test)
• create_research_workflow compiles with datastores= configured
• ruff / mypy / codespell / markdownlint / commitizen all pass

Goes into the next release cut (b12 or 1.0-rc, depending on the call).