refactor(terminal): cleanup deprecated flush logic (#6373)

Removes flush logic that was leftover to test against ratatui's flush
Cleaned up the flush logic so it's a bit more intent revealing.
DrawCommand now owns the Cells that it draws as this works around a
borrow checker problem.

codex-rs/tui/Cargo.toml

@@ -42,6 +42,7 @@ codex-ollama = { workspace = true }
 codex-protocol = { workspace = true }
 color-eyre = { workspace = true }
 crossterm = { workspace = true, features = ["bracketed-paste", "event-stream"] }
+derive_more = { workspace = true, features = ["is_variant"] }
 diffy = { workspace = true }
 dirs = { workspace = true }
 dunce = { workspace = true }