The biggest AI productivity gains inside large organisations in 2026 come from internal copilots — embedded, identity-aware assistants that read and write across the messy reality of enterprise data. This guide is the practical 2026 architecture: the five-plane reference design, end-to-end identity propagation with row-level retrieval ACLs, per-source knowledge with intent-aware routing, scope-limited action tools with staged approvals and idempotency, layered session/user/organisational memory, the evaluation loop most teams skip, the build/buy/platform calculus, and the rollout playbook that gets sustained adoption past 70 percent instead of stalling at 15.