feat(tui): keyboard navigation for user messages

[nexxeln]

summary

• add a keyboard focus mode for navigating user messages so message actions are no longer mouse-only
• a toggle command blurs the prompt and pushes a messages keymap mode; up/k and down/j move between user messages (first up jumps to the newest, down past the newest exits), enter opens the revert/copy/fork menu, r/c/f run them directly, i exits
• focused message is centered and highlighted while the others dim; the prompt greys out like the leader-pending state
• extract shared MessageActions (revert/fork/collect*) reused by both the existing click dialog and the new keyboard path
• the toggle keybind ships unbound (none) for now while we settle on the entry chord — bind messages_focus_toggle to try it

testing

• bun typecheck
• bun test test/cli/tui/keymap.test.tsx