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

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+"""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

examples/plugins/weather/__init__.py

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+"""Example: package plugin with third-party dependencies.
+
+Demonstrates:
+  - Package plugin structure
+  - Third-party deps via requirements.txt (uv venv auto-install)
+  - Keyword activation + global search
+  - Deep reload during development
+  - Cancellation checks for I/O
+  - External command support
+
+Usage:
+  cp -r examples/plugins/weather config/plugins/weather
+
+  Set your API key:
+    env = OPENWEATHER_API_KEY, your_key_here
+
+  Open spotlight and type "wth london" or just "london".
+  After editing, deep-reload:
+    fabric-cli exec modus1 'deep_reload weather'
+"""
+
+import os
+from typing import Any
+
+from window.spotlight.api import SearchResult, SpotlightPlugin
+
+try:
+    import requests
+except ImportError:
+    requests = None  # type: ignore[assignment]
+
+
+class WeatherPlugin(SpotlightPlugin):
+    id = "weather"
+    name = "Weather"
+    icon = "weather-clear-symbolic"
+    keywords = ["wth", "weather"]
+    searchable = True
+    priority = 75
+
+    def __init__(self, context):
+        super().__init__(context)
+        self._cache: dict | None = None
+
+    def initialize(self) -> None:
+        pass
+
+    def cleanup(self) -> None:
+        self._cache = None
+
+    def release_memory(self) -> None:
+        self._cache = None
+
+    def search(self, query: str, token: Any) -> list[SearchResult]:
+        q = query.strip().lower()
+
+        for prefix in ("wth ", "weather "):
+            if q.startswith(prefix):
+                q = q[len(prefix) :].strip()
+                break
+        if q in ("wth", "weather"):
+            q = ""
+
+        if not q:
+            return [
+                SearchResult(
+                    id="weather_hint",
+                    title="Weather",
+                    subtitle="Type 'wth <city>' to get current weather",
+                    icon_name="weather-clear-symbolic",
+                    score=100.0,
+                    render_type="default",
+                )
+            ]
+
+        data = self._fetch_weather(q, token)
+        if token.is_cancelled or not data:
+            return []
+
+        temp = data.get("main", {}).get("temp", "?")
+        desc = data.get("weather", [{}])[0].get("description", "?")
+        city = data.get("name", q)
+
+        return [
+            SearchResult(
+                id=f"weather_{q}",
+                title=f"{city}: {temp}°C",
+                subtitle=desc.capitalize(),
+                icon_name="weather-clear-symbolic",
+                score=100.0,
+                render_type="default",
+            )
+        ]
+
+    def _fetch_weather(self, city: str, token: Any) -> dict | None:
+        if self._cache:
+            return self._cache
+
+        api_key = os.environ.get("OPENWEATHER_API_KEY")
+        if not api_key or requests is None:
+            return None
+
+        try:
+            resp = requests.get(
+                "https://api.openweathermap.org/data/2.5/weather",
+                params={"q": city, "appid": api_key, "units": "metric"},
+                timeout=10,
+            )
+            if token.is_cancelled:
+                return None
+            if resp.status_code == 200:
+                self._cache = resp.json()
+                return self._cache
+        except Exception:
+            pass
+        return None
+
+    def handle_external(self, command: str, args: str) -> None:
+        print(f"[Weather] External: {command} {args}")
+
+    def on_activate(self, update_fn=None) -> None:
+        print("[Weather] Keyword mode activated")
+
+    def on_deactivate(self) -> None:
+        print("[Weather] Keyword mode deactivated")
+
+
+PLUGIN = WeatherPlugin

examples/plugins/weather/requirements.txt

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+requests>=2.28