fix: agent message flickering

[hugodutka]

Problem: agent messages often flicker when updated. Notice the extra lines that appear for a fraction of a second in the recording below.

Screen.Recording.2025-05-27.at.13.58.11.mov

It's because we sometimes capture the terminal screen while it's being redrawn and render an incomplete agent message, which appears to flicker.

This PR:

1. adds a kind of "vsync" that ensures the screen is stable before we capture it
2. fixes a race condition that sometimes causes an initial flicker right after a user message is posted
3. removes a hack to work around a bug in claude 0.2.70.

Points 1. and 2. should address the flickering, and 3. is maintenance work.