Feature: A secondary prefix (like tmux)

[blankjul]

Problem

herdr supports a single prefix key. tmux instead lets you bind two prefixes (prefix and prefix2), and a common ergonomic setup is one on each side of the keyboard. Whichever hand is free starts the chord, so the follow-up key is a shorter, more comfortable reach instead of stretching one hand across both prefix and action. With only one prefix, some prefix+key combos are awkward no matter where you put the single prefix.

Proposal

Add an optional keys.prefix2. Empty by default (disabled); when set, either the primary prefix or prefix2 enters prefix mode. No behavior change for anyone who doesn't set it.

Implementation

I've already built this and have been using it daily. Branch: https://github.com/blankjul/herdr/tree/prefix2

It mirrors the existing single-prefix flow (config field → validated_keybinds parse → is_prefix_key matches either combo), unifies the primary prefix into one KeyCombo for consistency, adds tests, and documents it under docs/next. just check passes.

Note

It looked like direct PRs from new contributors aren't desired without prior approval, so I'm opening this to check interest first rather than dropping a PR cold. Happy to open the PR whenever you'd like.