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/plugin_manager.gd

@@ -0,0 +1,154 @@
+extends Node
+class_name PluginManager
+## Scans res://plugins/ for plugin manifests (plugin.cfg) and provides
+## access to tile definitions. Instantiate tiles by tile_id.
+
+
+signal plugin_loaded(plugin_id: String, plugin_name: String)
+signal plugin_load_failed(plugin_id: String, reason: String)
+
+## All loaded plugin data: plugin_id -> Dictionary
+var _plugins: Dictionary = {}
+
+## All tile definitions: tile_id -> Dictionary
+var _tiles: Dictionary = {}
+
+## Plugin-id -> Array[tile_id] for reverse lookup
+var _plugin_tiles: Dictionary = {}
+
+
+func _ready() -> void:
+	_scan_plugins()
+
+
+## Scan res://plugins/*/plugin.cfg and load all manifests.
+func _scan_plugins() -> void:
+	var dir := DirAccess.open("res://plugins")
+	if dir == null:
+		return
+
+	dir.list_dir_begin()
+	var entry: String = dir.get_next()
+	while entry != "":
+		if entry.begins_with("."):
+			entry = dir.get_next()
+			continue
+		if dir.current_is_dir():
+			var manifest_path := "res://plugins/%s/plugin.cfg" % entry
+			if FileAccess.file_exists(manifest_path):
+				_load_manifest(entry, manifest_path)
+			else:
+				plugin_load_failed.emit(entry, "Missing plugin.cfg")
+		entry = dir.get_next()
+	dir.list_dir_end()
+
+
+func _load_manifest(plugin_id: String, path: String) -> void:
+	var cfg := ConfigFile.new()
+	if cfg.load(path) != OK:
+		plugin_load_failed.emit(plugin_id, "Failed to parse plugin.cfg")
+		return
+
+	var plugin_info := {
+		id = plugin_id,
+		name = cfg.get_value("plugin", "name", plugin_id),
+		version = cfg.get_value("plugin", "version", "0.1.0"),
+		description = cfg.get_value("plugin", "description", ""),
+		author = cfg.get_value("plugin", "author", ""),
+		base_path = path.get_base_dir(),
+	}
+	_plugins[plugin_id] = plugin_info
+
+	# Load tile definitions
+	var tile_count: int = cfg.get_value("tiles", "count", 0)
+	for i in range(tile_count):
+		var section := "tile_%d" % i
+		var tile_id: String = cfg.get_value(section, "id", "")
+		if tile_id.is_empty():
+			continue
+
+		# Scoped tile-id: "plugin_id/tile_id" ensures global uniqueness
+		var scoped_id := "%s/%s" % [plugin_id, tile_id]
+
+		if _tiles.has(scoped_id):
+			plugin_load_failed.emit(plugin_id, "Duplicate tile id: %s" % scoped_id)
+			continue
+
+		var tile_def := {
+			id = scoped_id,
+			plugin_id = plugin_id,
+			name = cfg.get_value(section, "name", tile_id),
+			scene = cfg.get_value(section, "scene", ""),
+			min_w = cfg.get_value(section, "min_w", 1),
+			min_h = cfg.get_value(section, "min_h", 1),
+			max_w = cfg.get_value(section, "max_w", 4),
+			max_h = cfg.get_value(section, "max_h", 4),
+			default_w = cfg.get_value(section, "default_w", 1),
+			default_h = cfg.get_value(section, "default_h", 1),
+		}
+		_tiles[scoped_id] = tile_def
+
+		if not _plugin_tiles.has(plugin_id):
+			_plugin_tiles[plugin_id] = []
+		_plugin_tiles[plugin_id].append(scoped_id)
+
+	plugin_loaded.emit(plugin_id, plugin_info.name)
+
+
+## Returns a list of all loaded plugin IDs.
+func get_plugin_ids() -> PackedStringArray:
+	var result: PackedStringArray = []
+	for pid in _plugins:
+		result.append(pid)
+	return result
+
+
+## Returns plugin info Dictionary for the given plugin_id, or null.
+func get_plugin(plugin_id: String) -> Dictionary:
+	return _plugins.get(plugin_id, {})
+
+
+## Returns all tile definitions across all plugins.
+func get_all_tile_defs() -> Array[Dictionary]:
+	var result: Array[Dictionary] = []
+	for tid in _tiles:
+		result.append(_tiles[tid])
+	return result
+
+
+## Returns a single tile definition by scoped tile_id, or null.
+func get_tile_def(tile_id: String) -> Dictionary:
+	return _tiles.get(tile_id, {})
+
+
+## Returns tile IDs belonging to a specific plugin.
+func get_plugin_tile_ids(plugin_id: String) -> PackedStringArray:
+	var result: PackedStringArray = []
+	for tid in _plugin_tiles.get(plugin_id, []):
+		result.append(tid)
+	return result
+
+
+## Instantiate a tile by its scoped tile_id.
+## Returns the root Control node, or null on failure.
+func instantiate_tile(tile_id: String) -> Control:
+	var def: Dictionary = _tiles.get(tile_id, {})
+	if def.is_empty():
+		return null
+	var scene_path: String = def.get("scene", "")
+	if scene_path.is_empty():
+		return null
+	var scene := load(scene_path) as PackedScene
+	if scene == null:
+		return null
+	var instance: Control = scene.instantiate() as Control
+	if instance == null:
+		return null
+
+	# Tag the instance so the layout manager can correlate it
+	if instance.has_method("set_tile_id"):
+		instance.set_tile_id(tile_id)
+	if instance.has_method("set_tile_config"):
+		instance.set_tile_config(def)
+
+	return instance