Constrain text selection to split pane

[edwin-zvs]

Summary

• make mouse drag text selection use the hovered split pane's inner bounds
• avoid selecting across sibling split panes when multiple main windows are visible
• keep the original whole-view fallback for non-split/no-window geometry cases

Tests

• cargo test -p agentd-cli selection_bounds_use_split_window_inner_area
• cargo test -p agentd-cli selection_bounds_use_view_inner_area
• cargo test -p agentd-cli find_text_ranges_respects_selection_bounds
• cargo check -p agentd-cli
• cargo build
• git diff --check

Binaries

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