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/__fish_help_header.fish

@@ -45,9 +45,10 @@ function __fish_help_header --argument-names name
     # First argument only -- see DESCRIPTION.
     contains -- "$argv[2]" -h --help; or return 1
 
-    set -l c_ttl (set_color --bold)
-    set -l c_sec (set_color --bold brblue)
-    set -l c_rst (set_color normal)
+    __fish_palette
+    set -l c_ttl $c_cmd
+    set -l c_sec $c_head
+    set -l c_rst $c_reset
     set -l miss "  No documentation header found. Try: help config $name"
 
     set -l file (functions -D -- $name 2>/dev/null)