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/arg0/src/lib.rs

@@ -54,7 +54,7 @@ pub fn arg0_dispatch() -> Option<TempDir> {
     match prepend_path_entry_for_codex_aliases() {
         Ok(path_entry) => Some(path_entry),
         Err(err) => {
-            // It is possible that Codex will proceed successfully even if
+            // It is possible that LLMX will proceed successfully even if
             // updating the PATH fails, so warn the user and move on.
             eprintln!("WARNING: proceeding, even though we could not update PATH: {err}");
             None