Franklin Agent 3.25.2 — keep text pastes instant

3.25.1's "always probe the clipboard" fix made every paste wait on the async
osascript / xclip / wl-paste shell-out before the text appeared, adding a
process-spawn delay to the common path (pasting code, URLs, prose).

• Text pastes are inserted synchronously again. The clipboard is now probed
  only when the bracketed-paste buffer looks like a terminal's image stub —
  empty (macOS image paste), or a single-line filename / file:// URI / raw
  image header (the Linux shapes from #77). Genuine text skips the probe
  entirely, so it lands instantly. The Linux image-paste fix from 3.25.1 is
  fully preserved. The stub heuristic is covered by unit tests.