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/dockup.fish

@@ -31,13 +31,13 @@ function dockup --description 'Pull and restart docker compose containers'
     # Handle help flags
     if contains -- -h $argv; or contains -- --help $argv
         __fish_palette
-        echo "$c_head""Usage:$c_reset $c_cmd""dockup$c_reset $c_dim""[DIRECTORY]$c_reset"
+        echo "$c_head""Usage:$c_reset $c_cmd""dockup$c_reset $c_arg""[DIRECTORY]$c_reset"
         echo ""
         echo "$c_head""Options:$c_reset"
         echo "  $c_flag-h$c_reset, $c_flag--help$c_reset    Show this help message"
         echo ""
         echo "$c_head""Arguments:$c_reset"
-        echo "  $c_dim""DIRECTORY$c_reset     Optional path to the compose project (defaults to current dir)"
+        echo "  $c_arg""DIRECTORY$c_reset     Optional path to the compose project (defaults to current dir)"
         return 0
     end