Script fs capability: add remove, mkdir, readFile, exists; frame-to-text helper

[kitlangton]

Drive 2.0.0 (464f9a3).

defineScript's fs exposes only writeFile. Removing a runtime-added plugin directory in plugin-registry.ts had to reach for node:fs/promises with a hand-built ${artifacts}/files/... path, which bypasses the project abstraction and will break if the files root ever moves.

Wish list, all relative to the project files root:

• fs.remove(path, { recursive? })
• fs.mkdir(path)
• fs.readFile(path)
• fs.exists(path)

A frame-to-text helper would also save every probe from re-deriving frame.lines.map(l => l.spans.map(s => s.text).join("")) when printing a screen on failure; ui.text() or a Frame.text(frame) export would do.