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/wake-lock.fish

@@ -22,6 +22,8 @@
 # EXAMPLE
 #   wake-lock rsync -avz src/ dest/
 function wake-lock --description 'Run a command while inhibiting system sleep'
+    __fish_help_header (status current-function) $argv; and return 0
+
     if test (count $argv) -eq 0
         set -l c_head (set_color --bold cyan)
         set -l c_cmd (set_color --bold)