fix: guard and document the external tools PR #168 flagged as unguarded

PR #168's Notes section named several functions with a real
external-tool dependency that no `type -q`/`command -q`/`command -v`/
`which` guard covers anywhere in the tree, deliberately left out of
DEPENDENCIES to avoid breaking test_dependencies_resolve. Adds the
guard each was missing, then declares the dependency now that it
resolves:

- bkg, detach: nohup
- gitui: gitui (self-shadow; type -q -f to skip the function itself)
- play-media: mpv, vlc -- already guarded via `type -q -f $p` in a
  loop, just never recognized as one (see next point)
- steam-dl: systemd-inhibit, steam
- wake-lock: systemd-inhibit
- split, spwin, tab: wezterm, konsole (kitty already declared)

docs/verify-manual.py's guard-detection regex only matched `type -q
<name>` immediately, so `type -q -f $p` (the `-f` flag excludes
functions from the match, needed wherever a wrapper shadows a binary
of its own name) was invisible to it -- both as a direct guard and
through the loop-variable indirection. Broadened both patterns to
skip over any flags between `-q` and the name/variable.

split/spwin/tab dispatch on $TERM/$TERM_PROGRAM/$KONSOLE_VERSION to
pick which terminal-specific binary to call, per this repo's C4
convention -- but those env vars only prove the terminal type, not
that its CLI binary is on $PATH: they propagate over ssh, so sshing
out from Kitty/WezTerm inherits the var on a remote host that never
installed the binary. Same latent gap in clone/clonet, whose
clone-in-kitty is a function Kitty's own shell integration injects,
not present on a remote shell that only inherited $TERM. All five now
check the actual thing they are about to call, not just the env var
that selects it.

Also guards and documents three more real, previously-undeclared
dependencies found by the same audit, unrelated to PR #168's named
list but the identical pattern: fast-cli (fast), lock (loginctl),
ports (lsof), screensleep (busctl).

docs/fish-config.md regenerated to match.

functions/steam-dl.fish

@@ -4,6 +4,9 @@
 # CATEGORY
 #   13-media-and-utilities
 #
+# DEPENDENCIES
+#   systemd-inhibit, steam
+#
 # SYNOPSIS
 #   steam-dl
 #
@@ -12,13 +15,21 @@
 #   or sleeping during active downloads.
 #
 # EXIT STATUS
-#   Exit status of steam (via systemd-inhibit)
+#   1  systemd-inhibit or steam is not installed
+#   *  Exit status of steam (via systemd-inhibit) otherwise
 #
 # EXAMPLE
 #   steam-dl
 function steam-dl --description 'Run Steam while inhibiting system sleep'
     __fish_help_header (status current-function) $argv; and return 0
 
+    for cmd in systemd-inhibit steam
+        if not type -q $cmd
+            echo (set_color red)"Error: $cmd is not installed."(set_color normal) >&2
+            return 1
+        end
+    end
+
     echo "Inhibiting sleep while Steam downloads..."
     systemd-inhibit --why="Active Download" --who="User" --what=idle:sleep steam
 end