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/split.fish

@@ -29,6 +29,10 @@
 #   split
 #   split -v nvim README.md
 function split --description 'Run a command in a new terminal split'
+    # -h is --horizontal here (see this function's own ARGUMENTS),
+    # so only the long form may reach the renderer.
+    test "$argv[1]" = --help; and __fish_help_header (status current-function) --help; and return 0
+
     # Opinionated guard (C4): integrations disabled
     if not __fish_config_op_enabled (status current-function)
         set -l c_err (set_color red)