fix: move_item must not overwrite advanced CRDT stage when missing_ok=true (bug 524)

When a story is found in the CRDT but not in the expected source stages,
and missing_ok is true, return Ok(None) instead of proceeding with the move.
This prevents promote_ready_backlog_stories from demoting a story that has
already advanced to merge/done via a stale filesystem shadow in 1_backlog.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/agents/lifecycle.rs

@@ -44,15 +44,23 @@ fn move_item<'a>(
 
         // Verify it's in one of the expected source stages.
         let src_dir = sources.iter().find(|&&s| current_dir == s).copied();
-        if src_dir.is_none() && !missing_ok {
-            let locs = sources
-                .iter()
-                .map(|s| format!("work/{s}/"))
-                .collect::<Vec<_>>()
-                .join(" or ");
-            return Err(format!("Work item '{story_id}' not found in {locs}."));
-        }
-        let src_dir = src_dir.unwrap_or(sources[0]);
+        let src_dir = match src_dir {
+            Some(s) => s,
+            None if missing_ok => {
+                // Item is in CRDT but not in an expected source stage — do not
+                // overwrite its current stage.  This prevents promote_ready_backlog_stories
+                // from demoting a story that has already advanced to merge/done (bug 524).
+                return Ok(None);
+            }
+            None => {
+                let locs = sources
+                    .iter()
+                    .map(|s| format!("work/{s}/"))
+                    .collect::<Vec<_>>()
+                    .join(" or ");
+                return Err(format!("Work item '{story_id}' not found in {locs}."));
+            }
+        };
 
         // Optionally clear front-matter fields from the stored content.
         let transform: ContentTransform = if fields_to_clear.is_empty() {