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

plugins/system_monitor/plugin.cfg

@@ -0,0 +1,41 @@
+[plugin]
+name="System Monitor"
+version="1.0.0"
+description="Monitors system resources: CPU and memory usage"
+author="Fifthdread"
+
+[tiles]
+count=3
+
+[tile_0]
+id="cpu"
+name="CPU Monitor"
+scene="res://plugins/system_monitor/tiles/cpu/cpu_tile.tscn"
+min_w=1
+min_h=1
+max_w=4
+max_h=4
+default_w=1
+default_h=1
+
+[tile_1]
+id="memory"
+name="Memory Monitor"
+scene="res://plugins/system_monitor/tiles/memory/memory_tile.tscn"
+min_w=1
+min_h=1
+max_w=4
+max_h=4
+default_w=1
+default_h=1
+
+[tile_2]
+id="testing"
+name="Testing"
+scene="res://plugins/system_monitor/tiles/testing/testing_tile.tscn"
+min_w=1
+min_h=1
+max_w=4
+max_h=4
+default_w=1
+default_h=1