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/application/src/agent/mod.rs

@@ -0,0 +1,25 @@
+use anyhow::Result;
+use std::future::Future;
+
+use zesdex_domain::agent::AgentTurnParams;
+
+/// Interface for dispatching tool calls to their concrete implementations.
+pub trait ToolExecutor: Send + Sync {
+    /// Execute a tool call asynchronously.
+    fn execute(
+        &self,
+        tool_name: &str,
+        args: &serde_json::Value,
+    ) -> impl Future<Output = Result<String>> + Send;
+}
+
+/// Service for running agent turns asynchronously.
+pub trait AgentTurnService: Send + Sync {
+    /// Run a full agent turn loop asynchronously.
+    fn run_turn(
+        &self,
+        params: AgentTurnParams,
+    ) -> impl Future<Output = Result<()>> + Send;
+}
+
+pub mod turn_service;