fix: default distributions to automatic GPU support with CPU fallback

AGENTS.md

@@ -9,6 +9,10 @@ readiness without completed release gates and recorded evidence.
 - All tests in the Windows VM must use CPU only. Never enable GPU providers here.
 - Production uses available GPU/hardware acceleration with reliable CPU fallback;
   the CPU-only rule is a VM testing constraint, not a product restriction.
+  Ship one normal hardware-detecting runtime per OS/architecture. Default
+  builds/staging use auto (Metal on macOS, Vulkan on Windows/Linux plus CPU).
+  Missing acceleration build SDKs must fail release preparation, never silently
+  produce CPU-only distributions. Explicit CPU builds remain internal VM tools.
 - Keep the Rust engine independent of the desktop shell; CLI and UI share its API.
 - Prefer established Rust crates and proven retrieval algorithms over custom
   implementations. Keep platform adapters small, bound allocations and queues,