epic: Phase 2 — Differentiate, go beyond #260

Goal

The SDK does things no generic gateway SDK does, because it understands this
gateway's governance model — and it integrates with, never duplicates, what the
platform has already shipped.

Scope

Refusal handlers, classification registry, HITL normalisation, thin identity
helpers, in-repo scanner + GitHub Action, kill-switch awareness, MCP tool
discovery, a second adapter by demand, and A2A serve/expose/dev.

Exit criteria

• At least one framework's official docs link to the SDK.
• Evidence of production use — a customer or internal team running it with
  OTel attached.
• Refusal handlers, registry, HITL and scanner all pass conformance against
  the simulator.

Standing constraint

Several features here are blocked on verification, and several deliberately
complement shipped platform capabilities rather than competing with them. Both
constraints are load-bearing, not caveats.