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/mod.rs

@@ -10,7 +10,7 @@ use crate::config::AgentConfig;
 use serde::{Deserialize, Serialize};
 
 pub use lifecycle::{
-    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,
     move_story_to_merge, move_story_to_qa, move_story_to_stage, reject_story_from_qa,
 };
 pub use pool::AgentPool;