[idea] Support tmux-style customizable keybindings (chord/prefix system) for 40% keyboards

[sodeyama]

idea / problem

herdr's keybinding system is mostly flexible — pane split/close, copy mode, focus movement, next/previous tab, and even the prefix key itself all accept arbitrary key combinations. But the "indexed" actions (switch_tab, switch_workspace, focus_agent) are hard-restricted to digit keys 1..9. Any other key assigned to them is silently rejected at parse time.

This is especially awkward on 40% keyboards, where the number row doesn't exist as a direct row — digits live behind a Fn/layer-shift combo instead of a single physical key. That makes tmux-style "prefix + number" jumps much more expensive to reach than on a full-size keyboard, since it requires holding an extra layer key on top of the prefix chord.

tmux doesn't have this limitation: bind-key lets you map window/pane-select actions to any key, so 40%-keyboard and non-QWERTY users commonly remap these to home-row keys (e.g. q w e r t or a s d f g) instead of the number row.

requested change

switch_tab = ["prefix+q", "prefix+w", "prefix+e", "prefix+r", "prefix+t"]

Position in the list would map to tab index. Keep 1..9 as the default so existing users see no change, with the current "prefix+1..9" shorthand remaining as sugar for the default ordered digit list.

Related: #1455 raises a similar underlying issue for resize-mode's hard-coded h/j/k/l directions (blocking Colemak-DH remaps). Both point at the same root cause — a handful of actions are parsed/validated as special-purpose types instead of going through the general keybinding path.

why you want this

I use a 40% keyboard, so number-row jumps are meaningfully slower for me than home-row jumps would be. Being able to remap switch_tab to keys next to the prefix chord would make tab switching as fast as the rest of herdr's keyboard-first workflow.

[magoz]

I have a related use case for focus_agent. With agent_panel_sort = "priority", I’d like to map F1, F2, F3, and F4 directly to priority-ranked agents 1–4.

Currently, focus_agent only accepts 1..9, while ordinary keybindings can use arbitrary keys. Supporting explicit key-to-index mappings would bring indexed actions closer to the rest of the keybinding system.

For example:

focus_agent_1 = "f1"
focus_agent_2 = "f2"
focus_agent_3 = "f3"
focus_agent_4 = "f4"

I’m not attached to that exact syntax—the important part is separating the destination index from the physical key. This would preserve dynamic priority ordering while allowing ergonomic layouts and avoiding terminal-wide remapping.

This also overlaps with the function-key request in #909.