fix(tui): preserve codex redraw frames in scrollback

[edwin-zvs]

Summary

• snapshot normal-screen Codex-style in-place redraw frames into the shadow scrollback before destructive/top-left redraws
• keep mouse wheel and C-x [ / C-x ] routing covered for Codex PTY views
• add regression coverage for pure TUI redraws that previously had no older frames to scroll to

Verification

• cargo test -p agentd-cli codex_ -- --nocapture
• cargo test -p agentd-cli many_pinned_sessions_resize_within_budget -- --nocapture
• cargo test -p agentd-cli
• cargo build
• git diff --check

Note: cargo fmt --check still reports broader pre-existing workspace formatting drift; touched CLI files were rustfmt'ed directly.