config(fish): add full fish config from live home; portable clipboard + env guards

Keep starship prompt (omp migration rejected). cpy/pst aliases branch on
wl-copy vs pbcopy; functions use the aliases. BROWSER/CHROME_EXECUTABLE
guarded on binary existence. auto-Niri.fish and cp-appid.fish are
linux-only (gated in .chezmoiignore).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

dot_config/fish/functions/mdcp.fish

@@ -3,6 +3,6 @@ function mdcp -a url
         echo "usage: mdcp <url>" >&2
         return 1
     end
-  curl.md $url | wl-copy
-  wl-paste | mdfried
+  curl.md $url | cpy
+  pst | mdfried
 end