Fix extension installation and plugin registration (fixes #8)

- Add Module.manifest marker file for proper extension recognition
- Fix UTF-8 em dash in extension.properties causing garbled description
- Register GhidraMCPPluginPackage so plugin appears in its own category
- Bump project_version to 0.2.1

Author: 13bm

Module.manifest

@@ -1,6 +1,6 @@
 name=GhidraMCP
-description=Model Context Protocol Server for Ghidra — 69 MCP tools for reverse engineering with AI
+description=Model Context Protocol Server for Ghidra - 69 MCP tools for reverse engineering with AI
 author=Brandon
 createdOn=03/22/2025
 version=12.0.4
-project_version=0.2.0
+project_version=0.2.1

extension.properties

@@ -0,0 +1,17 @@
+package ghidra.mcp;
+
+import javax.swing.Icon;
+
+import ghidra.framework.plugintool.util.PluginPackage;
+import resources.ResourceManager;
+
+public class GhidraMCPPluginPackage extends PluginPackage {
+
+    public static final String NAME = "GhidraMCP";
+
+    public GhidraMCPPluginPackage() {
+        super(NAME, ResourceManager.loadImage("images/package_green.png"),
+                "Model Context Protocol integration for Ghidra",
+                FEATURE_PRIORITY);
+    }
+}