tb-scripts: add ,optipng and ,zipdir utility scripts

Add two new shell utility scripts to the custom script packages:

,optipng - Parallel PNG optimization wrapper
- Detects available CPU cores and runs optipng in parallel using xargs
- Accepts optional file arguments, defaults to all *.png in current dir
- Uses maximum compression level (-o7) for best results
- Added to tb-dev-scripts package with optipng as runtime dependency

,zipdir - Quick directory archiving helper
- Creates a zip archive of the current directory
- Names the archive after the directory and places it in parent folder
- Uses maximum compression (-9) with extra attributes excluded (-X)
- Added to tb-scripts package with zip as runtime dependency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkgs/tb-dev-scripts/default.nix

@@ -20,5 +20,11 @@ pkgs.symlinkJoin {
         "--prefix PATH : ${lib.makeBinPath [ pkgs.ffmpeg ]}"
       ];
     } (builtins.readFile ./video_loop_extractor.py))
+
+    (pkgs.writeShellApplication {
+      name = ",optipng";
+      runtimeInputs = [ pkgs.optipng ];
+      text = builtins.readFile ./optipng;
+    })
   ];
 }