fix pane launch outside the plugin workspace; arrows place the pane

The pane command resolved its binary against the user's cwd, so the
pane only launched when that cwd was this repo. It now resolves from
$HERDR_PLUGIN_ROOT, which herdr injects into every plugin pane.

Arrow keys (or H/J/K/L) in the pane now move it left, below, above,
or right of your work pane. A second action, open-dagr-down, opens it
below directly.