Make EventPlanningCoordinator workflow more declarative and composable

[thegovind]

• The EventPlanningCoordinator pattern is problematic and indicates a gap in agent-framework.
• For simple use cases, the workflow should be small and ideally fully declarative.
• Consider a workflow that starts with the coordinator, fans out to specialists, then fans in to a reviewer, optionally looping back or summarizing.

Acceptance Criteria:

• EventPlanningCoordinator workflow is more declarative and composable.
• Example code and usage is simplified.
• Solicit feedback from Ben and Eric as needed.