v0.4.2

Divider fixes for editor terminals

• No more stacking dividers in VS Code — VS Code's shell integration rewrites the
  prompt around every command, which fed the old divider back into itself: one extra
  rule per command. The divider/marker now renders through a promptsubst call baked
  into PROMPT once — the prompt string never changes, so integrations wrap it a
  single time. Verified against VS Code's real shellIntegration-rc.zsh; also covers
  Cursor, Windsurf, and any other OSC 633-style shell integration.
• No more stranded divider rows on window resize outside tmux — the live divider
  redraw is now tmux-only (the Ghostty experience), where the repaint is reliable.
  Bare terminals (VS Code, JetBrains, kitty, …) simply draw the correct divider at
  the next prompt.
• Docs: README image refresh and formatting cleanups.