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

AGENTS.md

@@ -61,12 +61,22 @@ V Panel is a Godot Engine project that builds a fancy real-time status monitor.
 - All percentage labels get a 3px black outline (`outline_size` / `font_outline_color`) for legibility against the liquid shader background.
 - Title labels get a 2px outline.
 
+### Plugin System
+- `autoload/plugin_manager.gd` — autoload singleton, scans `res://plugins/*/plugin.cfg` at startup.
+- Each plugin folder contains a `plugin.cfg` INI file with `[plugin]` metadata and `[tile_N]` entries defining tiles (id, name, scene path, min/max grid spans).
+- Tile IDs are scoped as `{plugin_id}/{tile_id}` to guarantee global uniqueness.
+- `PluginManager.instantiate_tile(tile_id)` loads the tile scene, instantiates it, and tags the root node with `tile_id` and `tile_config` metadata.
+- `scripts/plugin_tile.gd` — class_name `PluginTile`, extends `ModuleBase`. Plugin tiles extend this instead of ModuleBase directly. Stores `tile_id` and `tile_config` properties, also written to node metadata for grid/layout access.
+
+### Layout System
+- `autoload/layout_manager.gd` — autoload singleton for saving/restoring tile grid positions.
+- Layout files stored as INI in `res://config/layouts/{name}.cfg`. Each entry maps a `tile_id` to `{col, row, w, h}`.
+- `save_layout()` writes the current in-memory layout to disk. `load_layout(name)` reads it back.
+- `switch_layout(name)` saves current layout, loads the named one, emits signals.
+- Current layout name persisted in `ConfigManager` as `layout_current`.
+- Layout auto-saves on every `module_placed` and `module_resized` signal from `DashboardGrid`.
+
 ### Config System
-- `autoload/config_manager.gd` — autoload singleton using Godot's built-in `ConfigFile` class for INI-style config files.
-- Two files: `res://config/default.cfg` (shipped defaults, tracked) and `res://config/config.cfg` (user overrides, gitignored). User overrides are merged on top of defaults per-key.
-- Keys are flattened as `{section}_{key}` for simple `get_setting(key, default)` lookups (e.g., `[background]` + `color` => `"background_color"`).
-- `get_color(key, default_color)` parses `"r, g, b"` string values from config into `Color`.
-- `set_setting(key, value)` diffs against defaults and writes only the diff to `config.cfg` automatically.
 
 ### Splash Screen
 - `scenes/splash.tscn` + `scenes/splash.gd` — entry point (set in `project.godot` as `main_scene`), handles fullscreen, font-size zoom animation, crossfade to dashboard.
@@ -83,16 +93,20 @@ res://
 │   ├── icons/
 │   └── textures/    # noise_100.png (tileable Perlin noise for shader distortion)
 ├── autoload/        # Singleton/autoload scripts
-│   └── config_manager.gd   # INI config via ConfigFile
+│   ├── config_manager.gd   # INI config via ConfigFile
+│   ├── plugin_manager.gd   # Plugin scanning, tile instantiation
+│   └── layout_manager.gd   # Layout save/load/switch
 ├── config/          # Configuration files (INI format)
 │   ├── default.cfg  # Shipped defaults, tracked in git
-│   └── config.cfg   # User overrides, gitignored (auto-generated)
-├── panels/          # Individual status panels (modules)
-│   ├── cpu/
-│   ├── memory/
-│   ├── network/
-│   ├── disk/
-│   └── testing/
+│   ├── config.cfg   # User overrides, gitignored (auto-generated)
+│   └── layouts/     # Saved layout files (*.cfg)
+├── plugins/         # Plugin folders
+│   └── system_monitor/  # Built-in system monitoring plugin
+│       ├── plugin.cfg
+│       └── tiles/
+│           ├── cpu/
+│           ├── memory/
+│           └── testing/
 ├── scenes/          # Root scenes
 │   ├── splash.tscn  # Animated splash → dashboard transition
 │   └── dashboard.tscn  # Main dashboard (PanelContainer root)
@@ -100,6 +114,7 @@ res://
 │   ├── dashboard_grid.gd   # Responsive grid with drag, resize, preset popup
 │   ├── module_base.gd      # Base class for all modules (mouse_filter IGNORE)
 │   ├── panel_base.gd       # Base class for panels
+│   ├── plugin_tile.gd      # Base class for plugin tiles (extends ModuleBase)
 │   └── shader_presets.gd   # Shader preset definitions and apply function
 ├── themes/          # Theme definitions and style resources
 ├── shaders/         # Custom shader materials