PR2 of 3 closing #71. Depends on TechDevGroup/devagentic#220 (graftedContextById query — merge that first).

Summary

• Extends plugins/devagentic-lane-h/client.py with fetch_grafted_context(graft_id, user_id=None) — wraps the new graftedContextById GraphQL query. Pattern mirrors existing fetch_reasoning_graft exactly (same plugin, same transport, same fail-soft contract).
• Registers grafted_context_fetch(graft_id, user_id?) MCP tool in mcp_serve.py::_register_lane_h_tools next to lane_h_fetch.
• 33 MCP tools registered total (was 32, +1 for grafted_context_fetch).

Why this plugin

The Lane H read pattern (G4 / hermes-agent#58) and the lazy-load preamble pattern (#71) are structurally identical — both fetch single per-user docs by id. Putting them in the same plugin keeps the worker's mental model simple (one "fetch a graft" surface, two doc kinds underneath).

Test plan

• 8 new tests in tests/plugins/devagentic-lane-h/test_grafted_context_fetch.py cover: empty graft_id guard, missing user_id guard, happy path, user_id arg override, null doc → actionable error, network error, GraphQL errors, non-dict response.
• 12 sibling tests in test_client.py still pass — no regression to the existing list_reasoning_grafts / fetch_reasoning_graft.
• create_mcp_server() smoke registers 33 tools; grafted_context_fetch + lane_h_fetch both present.

Next in series

• PR3: G1 preamble loader renders INDEX only ([graft_id | source | path | 1-line abstract]) instead of full graft bodies + injects "use grafted_context_fetch(graft_id) to load full content" instruction. Cuts ~30KB of per-turn preamble noise for verticals with many grafts; matches the saturation observation from G1 preamble doesn't scale past ~10 grafts even with caps — needs carving / lazy-load / summarization #71.

Notes

• user_id arg supports operator-side queries from a different profile (e.g. inspecting another vertical's grafts from your hermes session). Defaults to the active profile when omitted — the usual case.
• Server-side query enforces per-user scoping; passing the wrong user_id returns null rather than the doc. No cross-vertical leakage path exists from this client.