Add delete command for removing saved accounts

[Copilot]

Adds a dedicated delete command to remove saved account profiles, supporting both interactive and non-interactive modes (similar to use).

Changes

• New delete command (src/commands/delete.ts): Accepts optional account name argument; launches interactive selector if omitted
• AccountService.removeAccount(): Validates account exists, prevents deleting the active account, removes the account JSON file
• CannotDeleteActiveAccountError: Clear error when attempting to delete the currently active account
• README.md: Updated usage examples and command reference

Usage

# Delete by name
codex-auth delete my-account

# Interactive selection
codex-auth delete

Attempting to delete the active account returns an error prompting the user to switch first.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for a delete/remove command</issue_title>
<issue_description>First thank you for this small utility. Really helps when juggling between multiple OpenAI accounts.

Just wanted to recommend a small enhancement. We could add a dedicated remove/delete command to remove profiles. Bonus point if we can either do it interactively or not. Similar to the use command.

While we can still delete profiles manually by navigating to the .codex/accounts folder, having a dedicated command would make the user experience better.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add support for a delete/remove command #1

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