feat: add ADR-087 RuVix Cognition Kernel architecture

Defines a cognition kernel for the Agentic Age with 6 primitives
(task, capability, region, queue, timer, proof), 12 syscalls, and
RVF as the native boot object. Includes coherence-aware scheduler,
proof-gated mutation as kernel invariant, seL4-inspired capabilities,
io_uring-style queue IPC, 8 demo applications, and a two-phase build
path (Linux-hosted nucleus → bare metal AArch64).

Co-Authored-By: claude-flow <ruv@ruv.net>

Author: ruvnet