Fix UI text: Replace remaining "Codex" with "LLMX"

Updated user-facing strings throughout the TUI:

Slash commands:
- "instructions for Codex" → "instructions for LLMX"
- "ask Codex to undo" → "ask LLMX to undo"
- "exit Codex" → "exit LLMX"
- "what Codex can do" → "what LLMX can do"
- "log out of Codex" → "log out of LLMX"

Onboarding screens:
- "running Codex" → "running LLMX"
- "allow Codex" → "allow LLMX"
- "use Codex" → "use LLMX"
- "autonomy to grant Codex" → "autonomy to grant LLMX"
- "Codex can make mistakes" → "LLMX can make mistakes"
- "Codex will use" → "LLMX will use"

Chat composer:
- "Ask Codex to do anything" → "Ask LLMX to do anything"

Schema name:
- "codex_output_schema" → "llmx_output_schema"

Files changed: 7 files in TUI and core

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

llmx-rs/core/src/client_common.rs

@@ -371,7 +371,7 @@ pub(crate) fn create_text_param_for_request(
             r#type: TextFormatType::JsonSchema,
             strict: true,
             schema: schema.clone(),
-            name: "codex_output_schema".to_string(),
+            name: "llmx_output_schema".to_string(),
         }),
     })
 }
@@ -518,7 +518,7 @@ mod tests {
 
         assert_eq!(
             format.get("name"),
-            Some(&serde_json::Value::String("codex_output_schema".into()))
+            Some(&serde_json::Value::String("llmx_output_schema".into()))
         );
         assert_eq!(
             format.get("type"),