feat(core): add PluginDiscovery directory scan

src/AcDream.Core/Plugins/PluginDiscovery.cs

@@ -0,0 +1,38 @@
+namespace AcDream.Core.Plugins;
+
+public sealed record PluginDiscoveryResult(
+    string PluginDirectory,
+    PluginManifest? Manifest,
+    string? Error)
+{
+    public bool Success => Manifest is not null && Error is null;
+}
+
+public static class PluginDiscovery
+{
+    public static IReadOnlyList<PluginDiscoveryResult> Scan(string pluginsRootDirectory)
+    {
+        if (!Directory.Exists(pluginsRootDirectory))
+            return Array.Empty<PluginDiscoveryResult>();
+
+        var results = new List<PluginDiscoveryResult>();
+        foreach (var subdir in Directory.EnumerateDirectories(pluginsRootDirectory))
+        {
+            var manifestPath = Path.Combine(subdir, "plugin.json");
+            if (!File.Exists(manifestPath))
+                continue;
+
+            try
+            {
+                var json = File.ReadAllText(manifestPath);
+                var manifest = PluginManifest.Parse(json);
+                results.Add(new PluginDiscoveryResult(subdir, manifest, null));
+            }
+            catch (Exception ex)
+            {
+                results.Add(new PluginDiscoveryResult(subdir, null, ex.Message));
+            }
+        }
+        return results;
+    }
+}