Support a standalone master (judge) separate from worker slots

Foundation for the scout multi-agent flow: the master can now be its own agent + model that synthesizes the workers' reports without being one of the worker slots.

• settings: add ReviewProfileConfig.MasterAgent (standalone master); legacy worker Master still honored when MasterAgent is empty.
• profile: add profileMasterIdentity (MasterAgent wins, else legacy worker), masterDisplayLabel; resolveProfileMaster now delegates.
• cmd: multi-agent validation accepts a standalone master (validated for text generation) instead of requiring master ∈ workers; run path and scripted config updated accordingly.
• tests for master resolution precedence.