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

autoload/layout_manager.gd

@@ -0,0 +1,150 @@
+extends Node
+class_name LayoutManager
+## Manages saved dashboard layouts.
+##
+## Layouts map tile_id -> {col, row, w, h}. The current layout is loaded
+## on startup and auto-saved when modules are placed or resized.
+## Supports multiple named layouts with a simple file-per-layout scheme.
+
+
+signal layout_changed(layout_name: String)
+signal layout_saved(layout_name: String)
+signal layout_loaded(layout_name: String)
+
+const LAYOUT_DIR: String = "res://config/layouts/"
+
+## Current layout name (e.g. "default").
+var current_layout_name: String = "default"
+
+## The active layout data: tile_id -> {col, row, w, h}
+var _layout: Dictionary = {}
+
+
+func _ready() -> void:
+	if not DirAccess.dir_exists_absolute(LAYOUT_DIR):
+		DirAccess.make_dir_recursive_absolute(LAYOUT_DIR)
+
+	var configured_name: String = _get_configured_layout()
+	if not configured_name.is_empty():
+		current_layout_name = configured_name
+
+	if not load_layout(current_layout_name):
+		current_layout_name = "default"
+		_layout.clear()
+
+
+## Return the layout name from config, or empty.
+func _get_configured_layout() -> String:
+	if ConfigManager.has_method("get_setting"):
+		var val: Variant = ConfigManager.get_setting("layout_current", "")
+		if val is String and not val.is_empty():
+			return val
+	return ""
+
+
+## Set the current layout name (does not load or save).
+func set_current_layout(name: String) -> void:
+	current_layout_name = name
+	if ConfigManager.has_method("set_setting"):
+		ConfigManager.set_setting("layout_current", name)
+	layout_changed.emit(name)
+
+
+## Get tile_id -> {col, row, w, h} for the current layout.
+func get_layout() -> Dictionary:
+	return _layout.duplicate(true)
+
+
+## Set a single tile's position in the current layout.
+func set_tile_position(tile_id: String, col: int, row: int, w: int, h: int) -> void:
+	_layout[tile_id] = {col = col, row = row, w = w, h = h}
+
+
+## Remove a tile from the current layout.
+func remove_tile(tile_id: String) -> void:
+	_layout.erase(tile_id)
+
+
+## Save the current layout to config/layouts/{name}.cfg
+func save_layout(name: String = "") -> void:
+	if name.is_empty():
+		name = current_layout_name
+
+	var cfg := ConfigFile.new()
+	cfg.set_value("layout", "name", name)
+	cfg.set_value("layout", "saved_at", Time.get_datetime_string_from_system())
+
+	var tile_ids: Array = _layout.keys()
+	cfg.set_value("tiles", "count", tile_ids.size())
+	for i in tile_ids.size():
+		var tid: String = tile_ids[i] as String
+		var entry: Dictionary = _layout[tid]
+		var section := "tile_%d" % i
+		cfg.set_value(section, "id", tid)
+		cfg.set_value(section, "col", entry.get("col", 0))
+		cfg.set_value(section, "row", entry.get("row", 0))
+		cfg.set_value(section, "w", entry.get("w", 1))
+		cfg.set_value(section, "h", entry.get("h", 1))
+
+	var path := LAYOUT_DIR.path_join(name + ".cfg")
+	var result := cfg.save(path)
+	if result == OK:
+		current_layout_name = name
+		layout_saved.emit(name)
+
+
+## Load a saved layout, replacing the current in-memory layout.
+## Returns true if the file existed and was parsed.
+func load_layout(name: String) -> bool:
+	var path := LAYOUT_DIR.path_join(name + ".cfg")
+	if not FileAccess.file_exists(path):
+		return false
+
+	var cfg := ConfigFile.new()
+	if cfg.load(path) != OK:
+		return false
+
+	_layout.clear()
+	current_layout_name = name
+
+	var tile_count: int = cfg.get_value("tiles", "count", 0)
+	for i in range(tile_count):
+		var section := "tile_%d" % i
+		var tid: String = cfg.get_value(section, "id", "")
+		if tid.is_empty():
+			continue
+		_layout[tid] = {
+			col = cfg.get_value(section, "col", 0),
+			row = cfg.get_value(section, "row", 0),
+			w = cfg.get_value(section, "w", 1),
+			h = cfg.get_value(section, "h", 1),
+		}
+
+	layout_loaded.emit(name)
+	return true
+
+
+## Get a list of all saved layout names.
+func get_layout_names() -> PackedStringArray:
+	var dir := DirAccess.open(LAYOUT_DIR)
+	if dir == null:
+		return PackedStringArray()
+
+	var names: PackedStringArray = []
+	dir.list_dir_begin()
+	var entry: String = dir.get_next()
+	while entry != "":
+		if entry.ends_with(".cfg") and not entry.begins_with("."):
+			names.append(entry.trim_suffix(".cfg"))
+		entry = dir.get_next()
+	dir.list_dir_end()
+	return names
+
+
+## Switch to a different layout: saves current, loads new, emits signal.
+func switch_layout(name: String) -> void:
+	if name == current_layout_name:
+		return
+	save_layout(current_layout_name)
+	load_layout(name)
+	set_current_layout(name)