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_collector.gd

@@ -0,0 +1,51 @@
+extends RefCounted
+class_name CpuCollector
+
+
+var _previous_total: int = 0
+var _previous_idle: int = 0
+var _has_previous: bool = false
+
+
+func collect() -> float:
+    var stats: PackedStringArray = _read_stats()
+    if stats.is_empty():
+        return 0.0
+
+    var total: int = 0
+    var idle: int = 0
+
+    for i in range(1, stats.size()):
+        var val: int = stats[i].to_int()
+        total += val
+        if i == 4:  # idle is the 4th token (index 4)
+            idle = val
+
+    if not _has_previous:
+        _previous_total = total
+        _previous_idle = idle
+        _has_previous = true
+        return 0.0
+
+    var total_delta: int = total - _previous_total
+    var idle_delta: int = idle - _previous_idle
+
+    _previous_total = total
+    _previous_idle = idle
+
+    if total_delta == 0:
+        return 0.0
+
+    return 100.0 * (1.0 - float(idle_delta) / float(total_delta))
+
+
+func _read_stats() -> PackedStringArray:
+    var file := FileAccess.open("/proc/stat", FileAccess.READ)
+    if file == null:
+        return PackedStringArray()
+
+    var line: String = file.get_line()
+    if not line.begins_with("cpu "):
+        return PackedStringArray()
+
+    return line.split(" ", false)

plugins/system_monitor/tiles/cpu/cpu_tile.gd

@@ -0,0 +1,69 @@
+extends PluginTile
+
+
+@onready var title_label: Label = %Title
+@onready var label: Label = %Label
+@onready var vial_fill: ColorRect = %VialFill
+
+var _collector: CpuCollector
+var _displayed_fill: float = 0.0
+var _fill_tween: Tween
+
+
+func initialize() -> void:
+	module_title = "CPU"
+	_collector = CpuCollector.new()
+	_setup_shader()
+	_style()
+
+
+func refresh(data: Dictionary) -> void:
+	var usage: float = _collector.collect()
+	if usage < 0.0:
+		return
+
+	var pct: int = roundi(usage)
+	label.text = "%d%%" % pct
+
+	# Smoothly tween the vial fill
+	var target: float = usage / 100.0
+	if _fill_tween and _fill_tween.is_valid():
+		_fill_tween.kill()
+	_fill_tween = create_tween()
+	_fill_tween.tween_method(_set_fill, _displayed_fill, target, 0.4).set_ease(Tween.EASE_OUT).set_trans(Tween.TRANS_CUBIC)
+
+
+func _set_fill(value: float) -> void:
+	_displayed_fill = value
+	var mat := vial_fill.material as ShaderMaterial
+	if mat:
+		mat.set_shader_parameter("fill", value)
+
+
+func _setup_shader() -> void:
+	var mat := ShaderMaterial.new()
+	mat.shader = preload("res://shaders/vial_fill.gdshader")
+	mat.set_shader_parameter("liquid_color", Color(0.2, 0.5, 0.8, 1.0))
+	mat.set_shader_parameter("fill", 0.0)
+	mat.set_shader_parameter("noise_tex", load("res://assets/textures/noise_100.png"))
+	vial_fill.material = mat
+
+
+func _style() -> void:
+	# Transparent root panel — the VialFill shader provides the background
+	var panel := StyleBoxFlat.new()
+	panel.bg_color = Color.TRANSPARENT
+	panel.corner_radius_top_left = 12
+	panel.corner_radius_top_right = 12
+	panel.corner_radius_bottom_right = 12
+	panel.corner_radius_bottom_left = 12
+	add_theme_stylebox_override("panel", panel)
+	add_theme_stylebox_override("panel_focused", panel)
+
+	title_label.add_theme_color_override("font_color", Color(0.7, 0.7, 0.8, 1.0))
+	title_label.add_theme_constant_override("outline_size", 2)
+	title_label.add_theme_color_override("font_outline_color", Color(0.0, 0.0, 0.0, 0.5))
+	label.add_theme_color_override("font_color", Color(0.9, 0.9, 1.0, 1.0))
+	label.add_theme_font_size_override("font_size", 48)
+	label.add_theme_constant_override("outline_size", 3)
+	label.add_theme_color_override("font_outline_color", Color(0.0, 0.0, 0.0, 0.7))

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