Extract all cross-package type definitions and constructor functions
into a dedicated protocol layer. This is the foundation for the
seven-package split (RFC #143).
Contains:
- Result<T,E>, ok(), err()
- START, END constants
- CasStore, WorkflowFn, RoleOutput, WorkflowCompletion
- WorkflowDescriptor, WorkflowRoleDescriptor
- ProviderConfig, WorkflowConfig, ResolvedModel (fixes config↔registry cycle)
- RoleDefinition, Moderator, WorkflowDefinition
- AgentFn, ExtractFn, and all thread context types
Ref: #143, closes#144
