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/core/tests/suite/cli_stream.rs

@@ -126,7 +126,7 @@ async fn exec_cli_applies_experimental_instructions_file() {
     let custom_path_str = custom_path.to_string_lossy().replace('\\', "/");
 
     // Build a provider override that points at the mock server and instructs
-    // Codex to use the Responses API with the dummy env var.
+    // LLMX to use the Responses API with the dummy env var.
     let provider_override = format!(
         "model_providers.mock={{ name = \"mock\", base_url = \"{}/v1\", env_key = \"PATH\", wire_api = \"responses\" }}",
         server.uri()