docs(backends): make OS coverage explicit + require darwin for new backends

The backend matrix is the source of truth for which OS a backend ships on, but
that was never written down, so backends were landing Linux-only by default even
when the engine builds fine on macOS.

- .github/backend-matrix.yml: header block documenting the two matrices
  (include = Linux, includeDarwin = macOS/Apple Silicon) and the policy that new
  backends target every OS they can build for.
- .agents/adding-backends.md: a 'Cover every OS' subsection in step 2 (full darwin
  wiring: includeDarwin entry, index.yaml metal: + metal-<backend> entries,
  run.sh DYLD branch + inferBackendPathDarwin case for C++ backends, the
  hw_grpc_proto protobuf/grpc link gotcha, and the path-filter touch) plus a
  verification-checklist item.
- AGENTS.md (CLAUDE.md): Quick Reference pointer so it surfaces every session.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]

.github/backend-matrix.yml

@@ -2,6 +2,28 @@
 # Matrix data for backend container image builds.
 # Consumed by scripts/changed-backends.js for both backend.yml and backend_pr.yml.
 # This file is NOT a workflow — it has no top-level 'on:' or 'jobs:'.
+#
+# OS / platform coverage — READ THIS WHEN ADDING A BACKEND
+# --------------------------------------------------------
+# This file is the source of truth for which OS each backend is built and
+# published for. A backend ships ONLY for the matrices it appears in:
+#   - Linux  -> the `include:` matrix below (x86_64 + arm64; CPU and
+#               CUDA / ROCm / SYCL / Vulkan variants).
+#   - macOS  -> the `includeDarwin:` matrix (Apple Silicon / arm64; Metal where
+#               the engine supports it, otherwise a native arm64 CPU build).
+#
+# New backends must target EVERY OS they can build for, not just Linux. A backend
+# listed only under `include:` is silently unavailable on macOS even when its code
+# would run there. Most C/C++/GGML engines build on Darwin (ggml defaults
+# GGML_METAL=ON on Apple, so a plain build is Metal-enabled), and many Python
+# backends do too (CPU / MPS). If a backend genuinely cannot support an OS, say so
+# in its PR description rather than silently omitting it.
+#
+# Adding a backend to `includeDarwin:` is more than one line — see the darwin
+# checklist in .agents/adding-backends.md (includeDarwin entry, the index.yaml
+# `metal:` capability + `metal-<backend>` image entries, a `run.sh` Darwin/DYLD
+# branch for C/C++ backends, and the inferBackendPathDarwin case in
+# scripts/changed-backends.js so the path filter actually builds it).
 
 # Linux matrix (consumed by backend-jobs).
 include: