Expose model catalog and service-tier guidance through CLI and MCP

[galligan]

Context

Agents and humans need a cheap way to ask Dispatch what models and service tiers the current Codex App Server supports. This should come from the same op registry as the rest of Dispatch, not a hand-wired CLI-only command.

Scope

Expose a jq-friendly model/catalog read operation and project it through CLI and MCP.

Acceptance criteria

• A derived Dispatch op returns available models, service tiers, aliases, configured defaults, and source freshness.
• CLI output is stable JSON with --json and usable in scripts.
• MCP exposes the same operation from the same contract with read-only annotations.
• The surface helps agents choose valid model, effort, and service_tier combinations.
• dispatch schema works for the new command route.
• Docs/skill examples show how to inspect model/tier availability before launching a lane.