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/commands/InitCommand.kt

@@ -117,8 +117,10 @@ class InitCommand(
               # how many branches may build at the same time (at most one build per branch regardless)
               maxConcurrent: 1
 
-            # Build artifact retention.
+            # Build artifact storage and retention.
             artifacts:
+              # root directory for stored artifacts; empty = XDG_STATE_HOME (or ~/.local/state) + /gittally/artifacts/<repo-key>
+              rootDir: ""
               # number of builds to keep per branch
               retentionPerBranch: 3