feat(TUI): add autocomplete readline style keybinds

[timoclsn]

Add Ctrl+n/p navigation to autocomplete

Adds readline-style keybindings for navigating the autocomplete list when selecting files or agents with @.
What Changed

Autocomplete navigation:

• Added Ctrl+p (move up) and Ctrl+n (move down) as alternatives to arrow keys
• Keybinds only trigger when Ctrl is pressed alone (no Shift/Alt modifiers)
• Early returns prevent event re-handling
  Command keybind suspension:
• Fixed conflict where Ctrl+p would open command palette while navigating autocomplete
• Converted command dialog to use signal-based suspension with reference counting
• Global keybinds now properly respect the suspended state

I thin this should close #3379 after all.

closes: #3913