Unify master / restricted command gating in one helper

[rosspeili]

Problem

docs/ARCHITECTURE.md documents dual dispatch: main loop and handle_user_query() each enforce master-only lists (ask, markets, dna, send, 0x). Drift risk when new commands are added.

Goal

Single function, e.g. is_master_command(token) or assert_master_or_deny(user, cmd), used by both paths; unit tests for the matrix.

Acceptance criteria

• One definition of the restricted set; grep shows no duplicated literal lists (unless re-exported).
• Behavior unchanged for existing master and non-master profiles.