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

panels/cpu/cpu_module.gd

@@ -1,70 +0,0 @@
-extends ModuleBase
-class_name CpuModule
-
-
-@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))