fix(426): verify cherry-pick landed on master before marking story done

After the cherry-pick step in run_squash_merge, verify:
1. project_root is on the base branch (not a merge-queue branch)
2. HEAD commit has actual code changes (not an empty/story-only diff)

If either check fails, return success=false so the story stays in merge
stage for retry instead of being phantom-advanced to done.

Also rename move_story_to_archived → move_story_to_done.

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

server/src/agents/pool/lifecycle.rs

@@ -1729,7 +1729,7 @@ stage = "coder"
 
     #[tokio::test]
     async fn archiving_story_removes_agent_entries_from_pool() {
-        use crate::agents::lifecycle::move_story_to_archived;
+        use crate::agents::lifecycle::move_story_to_done;
         use std::fs;
 
         let tmp = tempfile::tempdir().unwrap();
@@ -1746,7 +1746,7 @@ stage = "coder"
 
         assert_eq!(pool.list_agents().unwrap().len(), 3);
 
-        move_story_to_archived(root, "60_story_cleanup").unwrap();
+        move_story_to_done(root, "60_story_cleanup").unwrap();
         pool.remove_agents_for_story("60_story_cleanup");
 
         let remaining = pool.list_agents().unwrap();

server/src/agents/pool/pipeline/advance.rs

@@ -308,7 +308,7 @@ impl AgentPool {
                             "[pipeline] Post-merge tests passed for '{story_id}'. Moving to done."
                         );
                         if let Err(e) =
-                            crate::agents::lifecycle::move_story_to_archived(&project_root, story_id)
+                            crate::agents::lifecycle::move_story_to_done(&project_root, story_id)
                         {
                             slog_error!("[pipeline] Failed to move '{story_id}' to done: {e}");
                         }

server/src/agents/pool/pipeline/merge.rs

@@ -104,7 +104,7 @@ impl AgentPool {
         }
 
         let story_archived =
-            crate::agents::lifecycle::move_story_to_archived(project_root, story_id).is_ok();
+            crate::agents::lifecycle::move_story_to_done(project_root, story_id).is_ok();
         if story_archived {
             self.remove_agents_for_story(story_id);
         }