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/hist.fish

@@ -17,6 +17,8 @@
 # EXAMPLE
 #   hist
 function hist --description 'Search fish history and put it in the prompt'
+    __fish_help_header (status current-function) $argv; and return 0
+
     # Opinionated guard (C4): integrations disabled
     if not __fish_config_op_enabled (status current-function)
         set -l c_err (set_color red)