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/tui/src/slash_command.rs

@@ -37,16 +37,16 @@ impl SlashCommand {
         match self {
             SlashCommand::Feedback => "send logs to maintainers",
             SlashCommand::New => "start a new chat during a conversation",
-            SlashCommand::Init => "create an AGENTS.md file with instructions for Codex",
+            SlashCommand::Init => "create an AGENTS.md file with instructions for LLMX",
             SlashCommand::Compact => "summarize conversation to prevent hitting the context limit",
             SlashCommand::Review => "review my current changes and find issues",
-            SlashCommand::Undo => "ask Codex to undo a turn",
+            SlashCommand::Undo => "ask LLMX to undo a turn",
             SlashCommand::Quit | SlashCommand::Exit => "exit LLMX",
             SlashCommand::Diff => "show git diff (including untracked files)",
             SlashCommand::Mention => "mention a file",
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::Model => "choose what model and reasoning effort to use",
-            SlashCommand::Approvals => "choose what Codex can do without approval",
+            SlashCommand::Approvals => "choose what LLMX can do without approval",
             SlashCommand::Mcp => "list configured MCP tools",
             SlashCommand::Logout => "log out of LLMX",
             SlashCommand::Rollout => "print the rollout file path",