skills: add GPU/A2A reference skill documents

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

skills/ref_lattice_vs_vector.md

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+---
+name: ref_lattice_vs_vector
+description: When to use Lattice<T> vs std::vector<T> for GPU-portable field storage in Grid
+metadata: 
+  node_type: memory
+  type: reference
+  originSessionId: 956e80aa-401d-481a-80bb-17f8abe1c131
+---
+
+## Rule
+
+Use `Lattice<vobj>` (or `std::vector<Lattice<vobj>>`) for any field that will be read or written inside `accelerator_for`. `std::vector<vobj>` is host memory and is NOT device-accessible.
+
+## Before vs after GPU offload
+
+```cpp
+// CPU-only (host memory, not GPU accessible)
+std::vector<SpinColourVector_v> tloopv(oSites, Zero());
+// accessed directly: tloopv[ss]
+
+// GPU-portable
+Lattice<SpinColourVector_v> tloop(grid);
+// accessed via view: autoView(tloop_v, tloop, AcceleratorWrite);
+//                    coalescedWrite(tloop_v[ss], val);
+```
+
+## Corollary: function signatures
+
+CPU-only version:
+```cpp
+void PackLeft(const std::vector<std::vector<vobj>> &leftv);
+```
+
+GPU-portable version:
+```cpp
+void PackLeft(const std::vector<Lattice<vobj>> &leftv);
+```
+
+## deviceVector for raw device buffers
+
+`deviceVector<T>` (defined in Grid) is like `std::vector<T>` but in device-accessible memory. Use for raw scalar scratch/pack buffers (e.g. GEMM input/output staging). Not for structured lattice data.
+
+## Pointer arrays for batched BLAS
+
+`deviceVector<scalar *>` holds batch pointer arrays. Populate with `acceleratorPut(ptrs[t], base + offset)` — sets device-side pointer from host. See `A2ASpatialSum::Allocate`.
+
+## Related
+[[ref_coalesced_views]] [[ref_accelerator_for]]