add plugin system with plugin/layout managers

- plugin_manager: scans res://plugins/*/plugin.cfg, loads tile definitions
- plugin_tile: new base class for plugin tiles, extends ModuleBase
- layout_manager: save/restore tile grid positions per named layout
- migrate existing cpu/memory/testing tiles into system_monitor plugin
- add module_resized signal to DashboardGrid for auto-save
- dashboard reads from PluginManager + LayoutManager instead of hardcoded paths
- project.godot registers PluginManager and LayoutManager autoloads
- AGENTS.md updated with new structure and conventions

scripts/plugin_tile.gd

@@ -0,0 +1,23 @@
+extends ModuleBase
+class_name PluginTile
+## Base class for all plugin tiles.
+##
+## Extends ModuleBase with plugin-aware metadata so the system can
+## correlate tile instances with their definitions and layouts.
+
+
+var tile_id: String = ""
+var tile_config: Dictionary = {}
+
+
+## Called by PluginManager.instantiate_tile() to tag this instance.
+func set_tile_id(value: String) -> void:
+	tile_id = value
+
+
+## Called by PluginManager.instantiate_tile() to pass the tile definition.
+func set_tile_config(config: Dictionary) -> void:
+	tile_config = config
+	# Store in meta so grid/layout can read it on any Control
+	set_meta("tile_id", tile_id)
+	set_meta("tile_config", config)