Summary

• add hit regions for split-window dividers during TUI render
• support left-click drag resizing for horizontal and vertical split dividers
• update split ratios during drag, including nested splits by render-order split id

Tests

• cargo test -p agentd-cli set_split_ratio_by_render_order_updates_nested_split
• cargo test -p agentd-cli split_windows_track_individual_pty_sizes
• cargo test -p agentd-cli mouse_scrollback_targets_only_hovered_split
• cargo check -p agentd-cli
• cargo build
• git diff --check

Binaries

Built in this worktree under /Users/moon/agentd/.claude/worktrees/tui-split-mouse-resize/target/debug/.
Relevant binary for this PR: agent (crates/cli).