Add support for the Magentic Orchestration Pattern

The current planner is a centralized agent-as-tool pattern. A single PlanningAgent owns the conversation, decides what to delegate, and invokes specialist agents as tools. Specialists do not coordinate directly with each other; they are exposed to the planner as callable units with bounded inputs. This keeps control flow simple, preserves the current interface contract, and makes startup/participant validation straightforward.

The planned **magentic** pattern is a peer-style orchestration model rather than a single-planner façade. Instead of one planning agent wrapping all specialists as tools, the orchestration layer would allow agents to participate more natively in a coordinated multi-agent exchange. That shifts the system from “planner delegates to tools” to “agents collaborate under an orchestration policy.” In practice, that means different routing semantics, different message/control flow, and likely different expectations around turn-taking, shared context, and coordination strategy. It is more flexible, but it also introduces more moving parts and a larger behavioral surface area than the current planner.