Add Phase 5 Kotlin desktop app: Compose Multiplatform with gRPC client

Kotlin/Compose Desktop application under desktop/ with:
- Gradle build: Kotlin 2.1, Compose 1.8, gRPC/Protobuf codegen, detekt
- ExoClient: gRPC client for both ArtifactService and KnowledgeGraphService
- Obsidian-style layout: collapsible tree sidebar, contextual main panel
- Five view modes: artifact detail, note editor, search results, catalog,
  graph (stub)
- Unified search bar with selector prefix support
- Command palette (Ctrl+Shift+A) for quick actions
- Menu bar with keyboard shortcuts (Ctrl+I/L/F/Q)
- Dark Material 3 theme
- Detekt lint config with Compose/test naming exceptions
- Unit tests for AppState and model types

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/settings.gradle.kts

@@ -0,0 +1,9 @@
+pluginManagement {
+    repositories {
+        google()
+        gradlePluginPortal()
+        mavenCentral()
+    }
+}
+
+rootProject.name = "exo-desktop"