Resize split PTYs to pane widths

[edwin-zvs]

Summary

• track actual inner PTY size per split window
• send per-split pty_resize dimensions for visible split sessions instead of one global right-pane size
• use the active split's size for hydration, new PTY sessions, and scroll-step calculations

Tests

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

Binaries

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