feat(tui): refine persistent terminal panes

[jlongster]

Summary

• refine terminal pane layout, headers, focus ownership, render timing, and live foreground-process labels
• add pane-navigation keybindings and prompt/session focus handling
• simplify pane layout state by removing the redundant BSP model
• handle click-only/empty selections while preserving current v2 copy-highlight behavior

Verification

• bun run typecheck in packages/tui and packages/cli
• bun test test/context/session-tabs.test.tsx test/feature-plugins/prompt-footer.test.tsx test/util/selection.test.ts in packages/tui (29 tests)

Stack: 5/5, based on #44836.