refactor: migrate to 100% Cortex backend (remove multi-provider support)

- Delete old provider files: backend.rs, openai.rs, openai_compatible.rs
- Replace client/mod.rs with simplified Cortex-only version
- Update cortex.rs with tool calling support from cortex-app
- Preserve validate_auth() method from cortex-cli improvements
- Update imports to use new client module structure

Author: Factory Bot

cortex-engine/src/agent/generator.rs

@@ -5,7 +5,7 @@
 //! what they want an agent to do, and the system will generate an appropriate
 //! agent configuration with system prompt, tool permissions, and metadata.
 
-use crate::client::{CompletionRequest, Message, create_backend_client};
+use crate::client::{CompletionRequest, Message, create_client};
 use crate::error::{CortexError, Result};
 use serde::{Deserialize, Serialize};
 
@@ -229,7 +229,7 @@ impl AgentGenerator {
 
     /// Generate an agent configuration from a natural language description.
     pub async fn generate(&self, description: &str) -> Result<GeneratedAgent> {
-        let client = create_backend_client(&self.model, self.backend_url.as_deref());
+        let client = create_client("cortex", &self.model, "", self.backend_url.as_deref())?;
 
         let request = CompletionRequest {
             messages: vec![