| Remove frames around text |

This should be fairly easy to implement: add --plain-text option and respect CLAUDE_PLAIN_TEXT environmental variable value to suppress generation of frames around prompt and some aspects of responses.

Rationale: Plain text is better when you wan to copy part of conversation (especially code) and insert it in other file/buffer/terminal.

This would help everyone and especially people using claude-code inside GNU Emacs (in vterm or eat mode) and other editors.

Top and bottom are not that annoying (can be replaced with "----" but
| left and right are pain. |