fix: prevent tmux flicker on restart by matching existing window size (#80)

When a PTY client re-attaches to an existing tmux session, it currently
hardcodes the PTY size to 120x40 and tmux resizes the window to match.
The xterm.js client then resizes back to its actual viewport on the
next render tick, so every restart causes a visible flicker and loses
one repaint of buffer content.

Also remove the hardcoded `-x 120 -y 40` from `tmux new-session` so
initial size adapts to the first client.

Changes:
- session.ts: query existing window size via `tmux display -p
  #{window_width} #{window_height}` before pty.spawn, fall back to
  120x40 only if tmux is unreachable.
- tmux-manager.ts: drop -x/-y from new-session args.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>