feat: add initial People Playground mod development kit

SledgeHammer/README.txt

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+SledgeHammer Mod
+================
+
+A basic example mod that introduces a classic sledgehammer weapon to People Playground.
+
+Features
+--------
+• Simple, functional sledgehammer item
+• Basic physics and collision detection
+• Demonstrates core modding concepts
+• Clean, minimal code structure
+
+How to Use
+----------
+1. Activate the mod in your mod list
+2. Find the sledgehammer in the weapons category
+3. Spawn it and start smashing!
+
+Technical Details
+-----------------
+This mod serves as an educational example showing:
+• Proper mod.json configuration
+• Item registration with ModAPI
+• Basic sprite loading and assignment
+• Simple weapon implementation
+
+Perfect for learning how to create your first People Playground mod.
+
+Credits
+-------
+Example mod for the People Playground Mod Development Project