"docs: add README"

README.md

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+# CommandHook
+
+Server-side mod for Vintage Story. Lets other mods hook into chat commands
+before and after they run, namespaced per mod so multiple mods can watch
+the same command without colliding.
+
+## How it works
+
+CommandHook patches `ChatCommandApi.Execute` and dispatches to whoever
+has registered. Your mod doesn't patch anything itself, it just implements
+`ICommandHookListener` and calls `CommandHookModSystem.Register`.
+
+A listener provides:
+
+- `ModId`, your mod's id. Used to namespace your registration so it doesn't
+  collide with another mod watching the same command
+- `Commands`, the command names you want to watch, without the leading slash
+- `Registration`, a `Before` delegate, an `After` delegate, or both
+
+`Before` runs before the command executes. Set `data.Cancel = true` inside it
+to stop the command from running. If you cancel, any remaining `Before`
+listeners for that command are skipped, and `After` never fires for that
+invocation.
+
+`After` runs once the command has produced a result. It only fires on the
+path where the command actually ran.
+
+`CommandData` is passed by `ref` the whole way through, no allocation per
+command.
+
+## Example
+
+```csharp
+public class MyListener : ICommandHookListener
+{
+    // Your mod's id, used to namespace this registration.
+    public string ModId => "mymod";
+
+    // Commands you want to watch, no leading slash.
+    public IReadOnlyList<string> Commands => new[] { "tp" };
+
+    // Wire up Before, After, or both.
+    public CommandRegistration Registration => new(Before, After);
+
+    private void Before(ref CommandData data)
+    {
+        // data.Sender is null for console invocations.
+        if (data.Sender != null && !IsAllowed(data.Sender))
+        {
+            // Stops the command from running and skips any remaining
+            // Before listeners. After never fires for this invocation.
+            data.Cancel = true;
+        }
+    }
+
+    private void After(ref CommandData data, TextCommandResult result)
+    {
+        // Only runs if nothing cancelled. result is whatever the command
+        // actually produced, check result.Status for success/error/deferred.
+        if (result.Status != EnumCommandStatus.Success)
+            Logger.Warn($"/{data.CommandName} failed: {result.StatusMessage}");
+    }
+}
+```
+
+Register it once your mod starts, unregister it on dispose:
+
+```csharp
+public override void StartServerSide(ICoreServerAPI api)
+{
+    // CommandHookModSystem.Instance is null if CommandHook isn't loaded,
+    // hence the ?. Register your listener once, here.
+    CommandHookModSystem.Instance?.Register(myListener);
+}
+
+public override void Dispose()
+{
+    // Always unregister on dispose, otherwise a stale listener stays in
+    // the dispatch table after your mod is gone.
+    CommandHookModSystem.Instance?.Unregister(myListener);
+}
+```
+
+Calling `Register` again with the same `ModId` replaces your existing
+registration. If your `Commands` list hasn't changed, it's a no-op.
+