Add Clide project structure and initial implementation

Set up the TUI IDE wrapper for Claude Code CLI with:
- Core app structure using Textual framework
- Panel architecture (sidebar, workspace, claude, context)
- Theme system with 22 built-in themes (Summer Night default)
- Pydantic models for configuration and data
- Makefile for development commands
- Project documentation and specs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/test_extensions.py

@@ -0,0 +1,52 @@
+"""Tests for extension system."""
+
+import pytest
+
+from clide.extensions import hookimpl
+from clide.extensions.manager import ExtensionManager
+
+
+class SampleExtension:
+    """Sample extension for testing."""
+
+    @hookimpl
+    def clide_on_app_startup(self, app: object) -> None:
+        """Track that startup was called."""
+        self.startup_called = True
+        self.received_app = app
+
+
+class TestExtensionManager:
+    """Tests for ExtensionManager."""
+
+    def test_register_plugin(self) -> None:
+        """Plugins can be registered manually."""
+        manager = ExtensionManager()
+        extension = SampleExtension()
+
+        manager.register_plugin(extension, "sample")
+
+        assert "sample" in manager.list_extensions()
+
+    def test_unregister_plugin(self) -> None:
+        """Plugins can be unregistered."""
+        manager = ExtensionManager()
+        extension = SampleExtension()
+        manager.register_plugin(extension, "sample")
+
+        manager.unregister_plugin("sample")
+
+        assert "sample" not in manager.list_extensions()
+
+    @pytest.mark.asyncio
+    async def test_trigger_startup_hook(self) -> None:
+        """Startup hooks are triggered for all extensions."""
+        manager = ExtensionManager()
+        extension = SampleExtension()
+        manager.register_plugin(extension, "sample")
+        mock_app = object()
+
+        await manager.trigger_app_startup(mock_app)
+
+        assert extension.startup_called is True
+        assert extension.received_app is mock_app