t_p6_api: build plugin scripting system

- Add PluginBase (plugin_base.gd) — abstract base class with 12 virtual hooks
- Add PluginManifest (plugin_manifest.gd) — custom Resource for plugin metadata
- Add PluginManager (plugin_manager.gd) — autoload singleton: directory scan,
  hook dispatch, lifecycle (load/unload/reload/rescan), cvar integration
- Add hello_world example plugin with all hooks demonstrating usage
- Register PluginManager autoload in project.godot
- Extend rcon_command_handler.gd with 'plugin' subcommand (list/load/unload/
  reload/info/rescan) delegating to PluginManager

server/plugins/hello_world/plugin.tres

@@ -0,0 +1,17 @@
+[gd_resource type="Resource" format=3]
+
+[ext_resource type="Script" path="res://server/scripts/plugin_api/plugin_manifest.gd" id="1_plugin_manifest"]
+
+[resource]
+script = ExtResource("1_plugin_manifest")
+name = "Hello World"
+version = "1.0.0"
+author = "Tactical Shooter Team"
+script_path = "hello_world.gd"
+enabled = true
+min_api_version = "1.0.0"
+max_api_version = ""
+metadata = {
+	"description": "Example plugin that logs all server events",
+	"url": "https://gitea.ourpad.casa/shawn/tactical-shooter"
+}