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.

functions/jr.fish

@@ -0,0 +1,28 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# CATEGORY
+#   08-terminal-management
+#
+# DEPENDENCIES
+#   jobrunner
+#
+# SYNOPSIS
+#   jr [<subcommand>] [<name>] [<command>...]
+#
+# DESCRIPTION
+#   Shorthand for jobrunner. Accepts the same subcommands, flags, and
+#   shorthands, and inherits its completions.
+#
+# ARGUMENTS
+#   See jobrunner --help for the full argument reference.
+#
+# EXIT STATUS
+#   Same as jobrunner.
+#
+# EXAMPLE
+#   jr run build make -j8
+#   jr list
+function jr --wraps jobrunner --description 'Shorthand for jobrunner'
+    jobrunner $argv
+end