Preserve spacing in Codex scrollback snapshots

[edwin-zvs]

Summary

Preserves meaningful blank rows in Codex/PTY shadow scrollback snapshots.

Before this change, the shadow snapshot path dropped every empty row before appending a repaint frame into scrollback. Older Codex pages therefore looked like a compact list of text, with visual spacing between user/assistant/tool sections collapsed.

Now the snapshot path:

• records text rows and internal blank rows,
• trims only leading/trailing blank padding,
• keeps duplicate snapshot detection intact.

Testing

• cargo test -p agentd-cli codex_shadow_snapshot_preserves_internal_blank_rows -- --nocapture
• cargo test -p agentd-cli codex_scrollback -- --nocapture
• cargo test -p agentd-cli codex_pure_tui_redraw_scrollback_exposes_older_frames -- --nocapture
• cargo check -p agentd-cli
• cargo build
• git diff --check