B8 — Decision record: core keeps first-class note types (non-goal guard)

[KrasForge]

Part of EPIC B0 #293

Depends on: —

Context

A well-meaning future PR (or agent) will try to generalize the core into a typeless property graph with note as just another plugin-registered type. That dissolves the concreteness the engine depends on and lands us in the "generic local-first app platform" trap. Forbid it explicitly.

Scope — in

• An ADR in docs/ stating: note/link/tag/user remain first-class core types; the node catalog's built-in type system is not moved to plugins; the core is not generalized into a domainless graph.
• Link the ADR from EPIC B0 EPIC B0 — Core/experience boundary refactor #293 and from CONTRIBUTING.

Scope — out

• No code changes.

Acceptance

• ADR merged and linked from the epic and CONTRIBUTING.
• B9's guard references it as rationale.