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

project.godot

@@ -0,0 +1,32 @@
+; Engine configuration file.
+; It's best edited using the editor UI and not directly,
+; since the parameters that go here are not all obvious.
+;
+; Format:
+;   [section] ; section goes between []
+;   param=value ; assign values to parameters
+
+config_version=5
+
+[application]
+
+config/name="Tactical Shooter"
+config/description="Phase 0 — Headless Dedicated Server"
+run/main_scene="res://scenes/server/server_main.tscn"
+config/features=PackedStringArray("4.7", "Forward Plus")
+
+[autoload]
+
+NetworkManager="*res://scripts/network/network_manager.gd"
+ServerConfig="*res://scripts/config/server_config.gd"
+PluginManager="*res://server/scripts/plugin_api/plugin_manager.gd"
+
+[physics]
+
+common/physics_ticks_per_second=128
+common/enable_pause_aware_picking=true
+
+[rendering]
+
+renderer/rendering_method="vulkan"
+environment/default_clear_color=Color(0.1, 0.1, 0.15, 1)