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/tiles/cpu/cpu_tile.tscn

@@ -0,0 +1,41 @@
+[gd_scene format=3 uid="uid://bq3bs2hb4r7fb"]
+
+[ext_resource type="Script" path="res://plugins/system_monitor/tiles/cpu/cpu_tile.gd" id="1"]
+[ext_resource type="Shader" path="res://shaders/vial_fill.gdshader" id="2"]
+
+[node name="CpuTile" type="PanelContainer"]
+anchors_preset = 0
+script = ExtResource("1")
+
+[node name="VialFill" type="ColorRect" parent="."]
+unique_name_in_owner = true
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+mouse_filter = 2
+
+[node name="MarginContainer" type="MarginContainer" parent="."]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+theme_constant_overrides/margin_left = 12
+theme_constant_overrides/margin_top = 12
+theme_constant_overrides/margin_right = 12
+theme_constant_overrides/margin_bottom = 12
+
+[node name="VBoxContainer" type="VBoxContainer" parent="MarginContainer"]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+
+[node name="Title" type="Label" parent="MarginContainer/VBoxContainer"]
+unique_name_in_owner = true
+text = "CPU"
+
+[node name="Spacer" type="Control" parent="MarginContainer/VBoxContainer"]
+size_flags_vertical = 3
+
+[node name="Label" type="Label" parent="MarginContainer/VBoxContainer"]
+unique_name_in_owner = true
+horizontal_alignment = 1
+size_flags_horizontal = 4