feat(help): standardize colored --help output across functions

Add the established c_head/c_cmd/c_flag/c_dim/c_arg color scheme to
--help (or usage-on-error) output in play-media and 13 other functions
that lacked it or used an ad hoc scheme: bkg, detach, replay, p, y,
spark, wake-lock, open-url, repo-open, dng2avif, dockup, fish-deps
(__fish_deps_help), and scrub.

Also tweak the standard itself:
- c_cmd now uses plain `set_color --bold` instead of `--bold white`,
  so the command name adapts to the terminal's foreground instead of
  forcing white text that washes out on light-background themes.
  Applied across all functions already using the pattern.
- jobrunner's reset variable renamed from c_rst to c_reset to match
  the naming used everywhere else.

functions/pkg.fish

@@ -47,7 +47,7 @@ function pkg --description 'Install or remove packages via the system package ma
 
     # ── Colour palette ───────────────────────────────────────────
     set -l c_head (set_color --bold cyan)
-    set -l c_cmd (set_color --bold white)
+    set -l c_cmd (set_color --bold)
     set -l c_flag (set_color yellow)
     set -l c_ok (set_color green)
     set -l c_warn (set_color yellow)