Complete comprehensive Codex → LLMX branding update

Fixed all remaining user-facing "Codex" references across entire codebase:
- Updated all UI strings and error messages
- Fixed GitHub issue templates and workflows
- Updated MCP server tool descriptions and error messages
- Fixed all test messages and comments
- Updated documentation comments
- Changed auth keyring service name to "LLMX Auth"

Reduced from 201 occurrences to only code identifiers (struct/type names).
Changes span 78 files across Rust, Python, YAML, and JSON.

🤖 Generated with Claude Code

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

llmx-rs/app-server/tests/suite/create_conversation.rs

@@ -26,7 +26,7 @@ async fn test_conversation_create_and_send_message_ok() -> Result<()> {
     let responses = vec![create_final_assistant_message_sse_response("Done")?];
     let server = create_mock_chat_completions_server(responses).await;
 
-    // Temporary Codex home with config pointing at the mock server.
+    // Temporary LLMX home with config pointing at the mock server.
     let codex_home = TempDir::new()?;
     create_config_toml(codex_home.path(), &server.uri())?;