feat(help): header-driven --help for terminal and session functions

spwin, tab, hist, bkg, wake-lock and fc take the standard first-arg-only
call site, so `wake-lock rsync --help` still runs rsync.

split takes --help only: its own ARGUMENTS documents -h as --horizontal,
so a literal test replaces the renderer's own flag check. Verified that
`split -h` still selects a horizontal split and never prints help.

functions/bkg.fish

@@ -23,6 +23,8 @@
 # EXAMPLE
 #   bkg firefox
 function bkg --description 'Execute bkg'
+    __fish_help_header (status current-function) $argv; and return 0
+
     # Check if a command was provided as an argument.
     if test -z "$argv[1]"
         set -l c_head (set_color --bold cyan)