implemented 05-artifact-store.md: filesystem artifact store with legacy-compatible naming, atomic persist from concurrent builds, retention pruning, and artifacts.rootDir config

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/kotlin/de/hoennig/gittally/artifacts/ArtifactsConfiguration.kt

@@ -0,0 +1,17 @@
+package de.hoennig.gittally.artifacts
+
+import de.hoennig.gittally.build.ArtifactStore
+import de.hoennig.gittally.config.ConfigLoader
+import org.springframework.context.annotation.Bean
+import org.springframework.context.annotation.Configuration
+
+@Configuration
+class ArtifactsConfiguration {
+    /**
+     * Store relative to the working directory, matching how `ConfigLoader` and the
+     * `BuildResultRepository` bean resolve their files. Nothing is touched until the
+     * first build persists, so the bean is safe outside a git repository.
+     */
+    @Bean
+    fun artifactStore(configLoader: ConfigLoader): ArtifactStore = FileArtifactStore(configLoader)
+}