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/fc.fish

@@ -24,6 +24,8 @@
 #   fc
 #   fc git
 function fc --description 'Edit and execute the last command (Bash-style fc)'
+    __fish_help_header (status current-function) $argv; and return 0
+
     set -l tmpfile (mktemp /tmp/fish_fc.XXXXXX).fish
 
     if count $argv >/dev/null