feat(spotlight): add plugin system with core engine and examples

- Plugin API (plugin.py, context.py, result.py)
- Core engine (search.py, manager.py, registry.py, loader.py)
- Built-in plugins: app, calculator, clipboard, emoji, power_menu, query, wallpapers
- Spotlight docs and example plugins
- Spotlight CSS styling

examples/plugins/hello.py

@@ -0,0 +1,46 @@
+"""Example: simplest possible single-file plugin.
+
+Usage:
+  cp examples/plugins/hello.py config/plugins/hello.py
+
+  Then open spotlight and type "hi" or "hello".
+  Or deep-reload from terminal:
+    fabric-cli exec modus1 'deep_reload hello'
+"""
+
+from typing import Any
+
+from window.spotlight.api import SearchResult, SpotlightPlugin
+
+
+class HelloPlugin(SpotlightPlugin):
+    id = "hello"
+    name = "Hello"
+    icon = "face-smile-symbolic"
+    keywords = ["hi", "hello"]
+    searchable = True
+    priority = 100
+
+    def initialize(self) -> None:
+        pass
+
+    def cleanup(self) -> None:
+        pass
+
+    def search(self, query: str, token: Any) -> list[SearchResult]:
+        if not query.strip():
+            return []
+        return [
+            SearchResult(
+                id="hello_world",
+                title="Hello, World!",
+                subtitle="A friendly greeting",
+                icon_name="face-smile-symbolic",
+                score=100.0,
+                render_type="default",
+                action=lambda: print("Hello!"),
+            )
+        ]
+
+
+PLUGIN = HelloPlugin