feat(tui): delay background tool hint

[nexxeln]

summary

• wait 3 seconds before showing the background-tool shortcut
• place the compact hint in the existing running-status row to avoid transcript reflow
• preserve elapsed time across remounts and reset the timer when the active tool changes

testing

• bun typecheck in packages/tui
• bun test in packages/tui (609 passed, 5 skipped)
• repository pre-push typecheck (33 tasks passed)