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/http/mcp/story_tools.rs

@@ -1,5 +1,5 @@
 use crate::agents::{
-    close_bug_to_archive, feature_branch_has_unmerged_changes, move_story_to_archived,
+    close_bug_to_archive, feature_branch_has_unmerged_changes, move_story_to_done,
 };
 use crate::http::context::AppContext;
 use crate::http::workflow::{
@@ -246,7 +246,7 @@ pub(super) fn tool_accept_story(args: &Value, ctx: &AppContext) -> Result<String
         ));
     }
 
-    move_story_to_archived(&project_root, story_id)?;
+    move_story_to_done(&project_root, story_id)?;
     ctx.agents.remove_agents_for_story(story_id);
 
     Ok(format!(
@@ -1331,7 +1331,7 @@ mod tests {
             .output()
             .unwrap();
 
-        // Create story file in current/ so move_story_to_archived would work.
+        // Create story file in current/ so move_story_to_done would work.
         let current_dir = tmp.path().join(".storkit/work/2_current");
         std::fs::create_dir_all(&current_dir).unwrap();
         std::fs::write(