feat(functions): derive the shared help palette from the fish theme

__fish_palette's c_cmd/c_flag/c_arg/c_dim now read fish's own highlighter
variables (fish_color_command/option/param/autosuggestion) when set, and
c_head reads the pager's fish_pager_color_prefix, so --help text matches
whatever the user's actual theme renders at the prompt instead of a fixed
guess at it. Each role keeps its previous static value as a fallback
(e.g. a --no-config script, or the theme variable being unset or empty).

Routes __fish_help_header's title/section colors and config-help's,
gi's, and git-clean's own hand-rolled --help blocks through the palette
instead of their hardcoded set_color calls, which fixes the
--help-vs-actual-prompt color mismatch across most of the config's
functions in one place (__fish_help_header backs the majority of
user-facing --help output).

Also fixes role mismatches found along the way: several already-palette
functions (dockup, fish-deps, pkg, play-media, dng2avif) were coloring
their <placeholder>/[bracket] argument text with c_dim instead of c_arg,
and scrub's "Aggressive Targets:" heading was still a hardcoded
set_color call sitting next to an otherwise-converted help block.

functions/git-clean.fish

@@ -29,9 +29,10 @@ function git-clean --description 'Sync main, prune remotes, and delete orphaned
     or return
 
     if set -q _flag_help
-        echo (set_color --bold blue)"Usage: "(set_color normal)"git-clean [OPTIONS]"
+        __fish_palette
+        echo "$c_head""Usage:$c_reset $c_cmd""git-clean$c_reset $c_arg""[OPTIONS]$c_reset"
         echo
-        echo "Steps taken:"
+        echo "$c_head""Steps taken:$c_reset"
         echo "  1. Fetches and prunes to find deleted remote branches."
         echo "  2. Switches to main if you are on an orphaned branch."
         echo "  3. Pulls the latest changes from the remote."