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.

completions/jobrunner.fish

@@ -0,0 +1,57 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# Completions for the `jobrunner` background job manager.
+# `jr` inherits these via `function jr --wraps jobrunner`.
+
+# Offer running jobs as name<TAB>description pairs.
+function __jobrunner_complete_jobs
+    for row in (__jobrunner_sessions)
+        set -l f (string split \t -- $row)
+        printf '%s\t%s job (PID %s)\n' $f[1] $f[3] $f[2]
+    end
+end
+
+set -l subcmds run list attach kill logs help
+set -l needs_job "__fish_seen_subcommand_from attach kill logs -a --attach -k --kill -o --output"
+
+# No file completions; jobs are named, not paths.
+complete -c jobrunner -f
+
+# Subcommands (only as the first argument).
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a run -d 'Start a named job in the background'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a list -d 'List all managed background jobs'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a attach -d 'Re-attach interactively to a job'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a kill -d 'Terminate a running background job'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a logs -d "Print a job's output without attaching"
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a help -d 'Show usage help'
+
+# Flag forms.
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -s r -l run -d 'Start a named job in the background'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -s l -l list -d 'List all managed background jobs'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -s a -l attach -d 'Re-attach interactively to a job'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -s k -l kill -d 'Terminate a running background job'
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -s o -l output -d "Print a job's output without attaching"
+complete -c jobrunner -s h -l help -d 'Show usage help'
+
+# Running jobs, for the subcommands that take one.
+complete -c jobrunner -n "$needs_job" -a '(__jobrunner_complete_jobs)'
+
+# A bare job name attaches to it, so offer jobs in first position too.
+complete -c jobrunner -n "not __fish_seen_subcommand_from $subcmds" \
+    -a '(__jobrunner_complete_jobs)'
+
+# After `run <name>`, complete the command to execute.
+complete -c jobrunner -n "__fish_seen_subcommand_from run -r --run; and test (count (commandline -opc)) -ge 3" \
+    -a '(__fish_complete_subcommand)'