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/scripts/rcon_command_handler.gd

@@ -11,6 +11,10 @@ extends Node
 #   help, echo/ping, status (engine-level stats), quit
 #   cvarlist, cvar_get, cvar_set (via CvarRegistry singleton if available)
 #
+# Plugin commands are delegated to PluginManager singleton:
+#   plugin list, plugin load, plugin unload, plugin reload,
+#   plugin info, plugin rescan
+#
 # Commands dispatched via signal:
 #   changelevel, kick, ban, unban, say, msg, players, exec
 #
@@ -51,6 +55,8 @@ func handle_command(cmd: String, args: PackedStringArray) -> String:
 			return _cmd_cvar_get(args)
 		"cvar_set":
 			return _cmd_cvar_set(args)
+		"plugin":
+			return _cmd_plugin(args)
 		"say", "msg", "players", "changelevel", "kick", "ban", "unban", "exec":
 			return _cmd_dispatch(cmd, args)
 		_:
@@ -76,6 +82,7 @@ func _cmd_help(_args: PackedStringArray) -> String:
 		"  cvarlist [pattern]          List cvars (optional filter)",
 		"  cvar_get <name>             Get a cvar's value",
 		"  cvar_set <name> <value>     Set a writable cvar",
+		"  plugin <subcommand> [args]  Plugin management (list/load/unload/reload/info/rescan)",
 		"  quit                        Shut down the server gracefully",
 	]
 	return "\r\n".join(lines) + "\r\n"
@@ -215,6 +222,20 @@ func _cmd_cvar_set(args: PackedStringArray) -> String:
 	
 	return "Cvar registry not available.\r\n"
 
+# ---------------------------------------------------------------------------
+# Plugin management — delegates to PluginManager singleton
+# ---------------------------------------------------------------------------
+func _cmd_plugin(args: PackedStringArray) -> String:
+	# Try to find PluginManager singleton (autoload)
+	var pm = get_node("/root/PluginManager")
+	if not pm:
+		return "PluginManager not available. Ensure PluginManager is registered as an autoload in project.godot.\r\n"
+
+	if not pm.has_method("rcon_command"):
+		return "PluginManager does not support RCON commands.\r\n"
+
+	return pm.rcon_command(args)
+
 # ---------------------------------------------------------------------------
 # Delegated commands — emit signal for game logic to handle
 # ---------------------------------------------------------------------------