Config-driven agent picker order (agent_order)

[aarontrowbridge]

Important

Problem — The agent picker order is fixed: the default agent first, then everything else alphabetically. Products that define a canonical mode sequence (plan, build, then their own director modes) cannot express it — custom primary agents get interleaved alphabetically and the intended sequence is unreachable without hardcoding names in the engine. The sequence must be expressible from config.
Approach — Add an optional top-level config key listing agents in their intended display order. Agent.list honors it as the primary sort; agents not named fall back to the current default-first-then-alphabetical behavior, and the key's absence preserves today's ordering exactly.
Scope — in: config schema + Agent.list sort · out: any product-specific agent names in the engine, TUI-side reordering
Assumptions — unknown names in the order key are ignored harmlessly; the default agent still floats to the top when it is not named in the key.

Acceptance Criteria

• With no order key configured, list order is unchanged (default agent first, then alphabetical).
• With an order key naming several agents, list() returns them in that sequence ahead of unlisted agents.
• Unknown names in the order key are ignored without error.
• The default agent floats to the top when it is not named in the order key.