feat(tui): expose bracketedPasteMode on PtyHandle
Add a readonly bracketedPasteMode: boolean to PtyHandle, parallel to the
existing alternateScreen / mouseMode / kittyKeyboardFlags getters.
Delegates to xterm-headless's native terminal.modes.bracketedPasteMode —
no manual CSI parser tracking needed.
Use case: pty-layout currently enables \\e[?2004h on its outer terminal
unconditionally so editors like helix get paste-no-indent. That forwards
paste markers to any focused pane regardless of whether the program
consuming stdin handles them; a CLI using gets() can receive literal ESC
sequences in pasted content. With this getter, pty-layout can proxy the
mode by-pane: when the focused pane has it on, write \\e[?2004h to outer;
when it doesn't, write \\e[?2004l.