added age-based build retention: artifacts.retentionMaxAge (e.g. 30d, empty = no limit) drops builds older than the given age; combines with retentionPerBranch as independent caps — a build is kept only while it satisfies both limits; a branch's newest build is never age-pruned and keepLatestGreen now shields the latest green build from both limits, keeping the permanent /branches/... links valid

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

src/main/kotlin/de/hoennig/gittally/watcher/Watcher.kt

@@ -250,7 +250,16 @@ class Watcher(
         originBranches: List<String>,
         workingDir: Path,
     ) {
-        repository.prune(originBranches, config.artifacts.retentionPerBranch, config.artifacts.keepLatestGreen)
+        val retentionCutoff =
+            config.artifacts.retentionMaxAge
+                .takeIf { it.isNotBlank() }
+                ?.let { clock.instant().minus(DurationParser.parse(it)) }
+        repository.prune(
+            originBranches,
+            config.artifacts.retentionPerBranch,
+            config.artifacts.keepLatestGreen,
+            retentionCutoff,
+        )
         artifactStore.prune(repository.history())
         pruneWorktrees(originBranches, workingDir)
     }