feat(jobrunner): add named background job manager backed by GNU screen

Adds jobrunner (short alias jr) for running, listing, inspecting,
re-attaching to, and terminating detached background tasks. Unlike bkg
and detach, which discard output, a jobrunner job keeps a live terminal
that survives closing the shell and can be restored later with attach.

Includes the __jobrunner_sessions parser shared by the function and its
completions, dynamic job-name completions, and screen in the fish-deps
catalog.

docs/fish-config.index

@@ -125,6 +125,8 @@ split=### split
 spwin=### spwin
 detach=### detach
 bkg=### bkg
+jobrunner=### jobrunner
+jr=### jr
 ssh=### ssh
 clipboard=## 5.9 Clipboard
 copy-fn=### y