Refactor subagent and workflow domain models; migrate access tiers and events to domain module

- Moved `AccessTier` and `SubagentEvent` enums to `zesdex_domain::subagent`.
- Consolidated workflow-related types into `zesdex_domain::workflow`.
- Updated references across the codebase to use the new domain models.
- Refactored tool execution logic to utilize a new `ToolExecutor` trait.
- Enhanced `AgentTurnService` to handle tool calls and events more effectively.
- Adjusted API handlers and state management to align with new domain structure.

apps/domain/src/lib.rs

@@ -28,6 +28,9 @@ pub mod auth;
 pub mod cms;
 pub mod core;
 pub mod error;
+pub mod agent;
+pub mod workflow;
+pub mod subagent;
 
 // Re-export all public items from each module for ergonomic imports.
 // Consumers can do `use zesdex_domain::*` for common types.
@@ -50,3 +53,6 @@ pub use core::{
     ToolCallResult, ToolDef, ToolFunction, ToolFunctionDef, UsageStats,
 };
 pub use error::DomainError;
+pub use agent::*;
+pub use workflow::*;
+pub use subagent::*;