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

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.

Start by inspecting the defineScript filesystem capability and plugin-registry.ts, especially the current writeFile use and direct node:fs/promises path handling. Trace the frame structure behind frame.lines and its spans before choosing the requested text helper. Done means the four filesystem operations work relative to the project files root and callers can obtain frame text without re-deriving the mapping.