IDE file/selection status now shown as a dismissible pill inside the prompt input

What

• The prompt input now shows IDE-selection status (the current file/selection from a connected editor) as a leadingPill embedded at the start of the input text, replacing the old separate sidebar box.
• The pill can be dismissed: a new dismissIdeSelection handler is wired to the pill's delete action, and a hash of the last-dismissed selection is tracked so the same selection doesn't reappear right after being dismissed.
• The underlying text input component gained a leadingPill prop, rendered before the main text with its own highlight color and correctly offsetting cursor position and highlight ranges.
• The text-editing logic (kill/yank, backspace, delete, Home/End, Enter) was reworked to treat this leading content as a protected prefix: backspace at the prefix boundary calls the pill's delete callback instead of erasing into it.

Why

This keeps the current file/selection context visible directly in the prompt instead of in a separate box, while preventing it from being accidentally deleted or re-shown right after being dismissed.