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

SledgeHammer/mod.json

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+{
+  "Name": "SledgeHammer",
+  "Author": "Nuclear.Lab",
+  "Description": "A heavy demolition sledgehammer with devastating impact force and realistic physics.",
+  "ModVersion": "1.0",
+  "GameVersion": "1.27.5",
+  "ThumbnailPath": "thumb.png",
+  "EntryPoint": "Mod.Mod",
+  "Tags": [
+    "Fun",
+    "Weapons",
+    "Melee"
+  ],
+  "Scripts": [
+    "script.cs"
+  ]
+}

SledgeHammer/script.cs

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+using UnityEngine; //You'll probably need this...
+
+namespace Mod
+{
+    public class Mod
+    {
+        public static void OnLoad()
+        {
+            // Optional initialization method - runs when mod is first loaded
+        }
+
+        public static void OnUnload()
+        {
+            // Optional cleanup method - runs when mod is unloaded/game closes
+        }
+
+        public static void Main()
+        {
+            // register item to the mod api
+            ModAPI.Register(
+                new Modification()
+                {
+                    OriginalItem = ModAPI.FindSpawnable("Power Hammer"), //item to derive from (better physics than Brick)
+                    NameOverride = "SledgeHammer", //new item name
+                    DescriptionOverride = "A heavy demolition sledgehammer.", //new item description
+                    CategoryOverride = ModAPI.FindCategory("Melee"), //new item category
+                    ThumbnailOverride = ModAPI.LoadSprite("sledgeHammerView.png"), //new item thumbnail (relative path)
+                    AfterSpawn = (Instance) => //all code in the AfterSpawn delegate will be executed when the item is spawned
+                    {
+                        Instance.GetComponent<SpriteRenderer>().sprite = ModAPI.LoadSprite("sledgeHammer.png"); //get the SpriteRenderer and replace its sprite with a custom one
+
+                        var physical = Instance.GetComponent<PhysicalBehaviour>();
+                        if (physical != null)
+                        {
+                            physical.InitialMass = 8f; //make it heavier for realistic sledgehammer feel
+                            physical.TrueInitialMass = 8f; //ensure the mass is actually applied
+                        }
+
+                        Instance.FixColliders(); //fix colliders to match new sprite
+                    }
+                }
+            );
+        }
+    }
+}