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/__jobrunner_sessions.fish

@@ -0,0 +1,43 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   __jobrunner_sessions
+#
+# DESCRIPTION
+#   Parses `screen -ls` into machine-readable rows, one per active session:
+#   name, PID, state, and start time separated by tabs. Shared by jobrunner
+#   and its completions so both agree on what a session is named. Prints
+#   nothing when no sessions exist.
+#
+# EXIT STATUS
+#   0  Parsed successfully (including the zero-session case)
+#   1  screen is not installed
+#
+# RETURNS
+#   One line per session: <name>\t<pid>\t<state>\t<started>
+#
+# EXAMPLE
+#   __jobrunner_sessions
+#   __jobrunner_sessions | string replace -r '\t.*$' ''
+function __jobrunner_sessions --description 'List active screen sessions as name/pid/state/started rows'
+    command -q screen; or return 1
+
+    # screen -ls exits 1 when nothing is running; the parse handles that.
+    for line in (command screen -ls 2>/dev/null)
+        # Session rows are tab-indented and start with "<pid>.<name>".
+        set -l m (string match -r '^\s+(\d+)\.([^\t]+)\t(.*)$' -- $line)
+        or continue
+
+        # Trailing fields are parenthesized: "(<date>)\t(<state>)", and some
+        # screen builds omit the date, so read the state off the end.
+        set -l fields (string split \t -- $m[4])
+        set -l state (string trim -c '()' -- $fields[-1])
+        set -l started ""
+        if test (count $fields) -gt 1
+            set started (string trim -c '()' -- $fields[1])
+        end
+
+        printf '%s\t%s\t%s\t%s\n' $m[3] $m[2] $state $started
+    end
+end