skills: add GPU/A2A reference skill documents

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

skills/ref_accelerator_for.md

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+---
+name: ref_accelerator_for
+description: Grid accelerator_for usage — converting block-strided thread_for to GPU-portable oSites loops
+metadata: 
+  node_type: memory
+  type: reference
+  originSessionId: 956e80aa-401d-481a-80bb-17f8abe1c131
+---
+
+## Pattern: block-strided thread_for → accelerator_for over oSites
+
+Old CPU-only pattern (block-strided over orthog dimension):
+```cpp
+thread_for(r, rd, {
+  int so = r * grid->_ostride[orthogdim];
+  for (int n = 0; n < e1; n++)
+  for (int b = 0; b < e2; b++) {
+    int ss = so + n * stride + b;
+    // work on site ss
+  }
+});
+```
+
+GPU-portable replacement:
+```cpp
+accelerator_for(ss, grid->oSites(), Nsimd, {
+  // work on site ss — one SIMT thread per (osite, lane) on GPU
+  // one thread per osite (lane loop implicit via GRID_SIMT) on CPU
+});
+```
+
+Key rules:
+- `accelerator_for(iter, count, Nsimd, body)` — Nsimd is `vobj::Nsimd()` or `grid->Nsimd()`
+- On CPU: expands to `thread_for` over count, `acceleratorSIMTlane` always returns 0 — must use `#ifdef GRID_SIMT` pattern if iterating lanes explicitly (see [[ref_grid_simt_pattern]])
+- On GPU: one SIMT thread per (iter × lane), `acceleratorSIMTlane(Nsimd)` returns actual lane
+- Loop body must capture only scalar/POD by value or via device-accessible pointers; no `std::vector` or host containers inside the body
+- `Coordinate` inside `accelerator_for` must be `AcceleratorVector<int, MaxDims>` (stack-allocated, device-safe) — Grid's `Coordinate` typedef already satisfies this
+
+## Where defined
+`Grid/threads/Accelerator.h` — CPU path ~line 607; GPU paths in conditional blocks above.
+
+## Model file
+`Grid/algorithms/blas/MomentumProject.h` — `ImportVector` is the canonical example of correct `accelerator_for` + SIMD lane extraction.