tmux: add keybinding command palette

[connorads]

Why

The tmux config already uses bind -N notes and a help popup. A searchable keybinding palette would make bindings discoverable in-context, without introducing another external plugin.

Scope

• Build a native tmux keybinding palette (no new tmux plugin dependency)
• Implement under existing dotfiles structure (e.g. ~/.config/tmux/scripts/)
• Support key tables currently in use:
  • prefix
  • root
  • copy-mode-vi
• Use existing binding notes (bind -N) as display labels where available
• Keep existing Ctrl+b ? help popup intact
• Add/adjust palette keybindings to avoid collisions with current bindings
• Update ~/.config/tmux/help.md with any new keys

Acceptance criteria

• Palette opens from configured keys
• Palette shows named bindings from current key table
• Selecting an entry triggers the expected tmux action
• Existing help popup (Ctrl+b ?) still works unchanged
• ~/.config/tmux/help.md reflects the new keys
• No additional third-party tmux plugin is required