Make the in-picker keybindings configurable

[mackt]

What

Every key inside herdr-plus's two TUIs is hardcoded. I'd love to be able to rebind them
from config.toml, the same way [worktree] branch_prefix is configurable today.

To be clear about scope: this isn't the [[keys.command]] binding covered in
docs/keybindings — that binds a
herdr key to launch the plugin. This is the keys inside the projects browser and the
quick-actions launcher, once they're open.

Current state

As of v0.1.20, in projectsmodel.go and quickactionspicker.go:

Keys	Action
up, ctrl+p	move highlight up
down, ctrl+n	move highlight down
enter	activate
esc	back / quit
ctrl+c	quit
ctrl+g	open as worktree (projects only)

Why

Muscle memory differs, and the current set has no vim-style pair — coming from vim,
ctrl+k/ctrl+j is what my fingers reach for, and it isn't bound to anything. Rather
than argue about which extra defaults to bake in, letting people declare their own seems
like the more general answer.

One constraint worth stating, since it shapes any design: plain j/k can never be
navigation here. Both pickers filter continuously, so every printable rune has to go to
the query. Only modified keys are available. (fzf has the same constraint, and ships
ctrl-j/ctrl-k for exactly this reason.)

Possible shape

Something like a [keys] section in the optional config.toml that already holds
[worktree]:

[keys]
up       = ["up", "ctrl+p", "ctrl+k"]
down     = ["down", "ctrl+n", "ctrl+j"]
select   = ["enter"]
cancel   = ["esc"]
worktree = ["ctrl+g"]

Anything not listed keeps today's behavior, so an absent [keys] changes nothing.

That's only meant to show what I'm asking for — does something along these lines fit how
you think about the plugin's config?