[Feature] Set a filetype for the claude-code buffer

## Feature Description

It would be convenient if this plugin set a filetype for the terminal that it opens

## Use Case

This makes it easier to target autocmds only to this specific window. E.g. I want to set `buflisted = false` for these, and this would make it possible without having to add more options to the plugin (though happy to add a `buflisted` option if you want)

No file or test is named in the issue. Locate the code that creates the Claude Code terminal buffer, set and verify the requested filetype there, and confirm that a filetype-specific autocmd can target only this buffer.

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