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)