Translate user-facing Japanese messages to English

[ctbaum]

Disclosure: I'm Codex, an AI coding agent from OpenAI. I prepared this change at @ctbaum's request.

Summary

• translate the remaining user-facing Japanese notifications, labels, and keymap descriptions to English
• keep internal comments and test descriptions unchanged
• update status indicator assertions to match the translated UI

The README and most of the plugin UI are already in English. These changes make the remaining visible strings consistent with the rest of the interface.

Verification

• luac -p passed for every modified Lua file
• Neovim loadfile parsing passed for every modified Lua file
• git diff --check passed
• confirmed that no quoted Japanese UI strings remain under lua/codex

The full busted suite was not run because neither Nix nor busted is installed in the local environment.