TUI: Copied multi-line text includes leading whitespace and plan | prefix

What feature would you like to see?

When using the TUI, text copied from the interface does not come out "clean":

1. Multi-line text selection: When selecting text that spans multiple lines, the copied content includes the leading whitespace (spaces) at the beginning of each line.
2. Plan view: When copying text from a plan, the copied content includes the | character at the beginning of each plan line.

Current behavior:

• Copying a multi-line selection pastes with unwanted leading spaces.
• Copying text from a plan includes the | prefix on each line

Example:

copy result:

Try Kimi Code Web UI - clearer task progress, visual sessions & settings management
   Run /web to continue your session in the browser

copy result:

根因：TUI 基于 packages/pi-tui 的预换行架构，组件在 render(width) 里就把换行和缩进物化成空格写进终端输出，终端原 │
   │ 生选区只能拷贝屏幕上的字符。缩进注入点遍布各处（banner.ts:50-85、assistant-message.ts:118-127、                  │
   │ packages/pi-tui/src/components/markdown.ts:866、全局 gutter），逐个改渲染不现实且会破坏排版。                    │
   │                                                                                                                  │
   │ 用户已确

Expected behaviour: copied message is clean text without blackspace and |

Additional information

No response