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/merge.rs

@@ -383,6 +383,71 @@ pub(crate) fn run_squash_merge(
         });
     }
 
+    // ── Verify code landed on the correct branch ──────────────────
+    // Guard against the cherry-pick silently landing on the wrong branch
+    // (e.g. a merge-queue branch from a concurrent merge).  If the current
+    // branch is not the base branch, or the HEAD commit has no code diff,
+    // treat the merge as failed so the story stays in the merge stage.
+    let current_branch = Command::new("git")
+        .args(["rev-parse", "--abbrev-ref", "HEAD"])
+        .current_dir(project_root)
+        .output()
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .unwrap_or_default();
+
+    let base_branch = crate::config::ProjectConfig::load(project_root)
+        .ok()
+        .and_then(|c| c.base_branch.clone())
+        .unwrap_or_else(|| "master".to_string());
+
+    if current_branch != base_branch {
+        all_output.push_str(&format!(
+            "=== VERIFICATION FAILED: expected branch '{base_branch}' but HEAD is on \
+             '{current_branch}'. Cherry-pick landed on wrong branch. ===\n"
+        ));
+        cleanup_merge_workspace(project_root, &merge_wt_path, &merge_branch);
+        return Ok(SquashMergeResult {
+            success: false,
+            had_conflicts,
+            conflicts_resolved,
+            conflict_details: Some(format!(
+                "Cherry-pick landed on '{current_branch}' instead of '{base_branch}'"
+            )),
+            output: all_output,
+            gates_passed: true,
+        });
+    }
+
+    // Verify HEAD commit has actual code changes (not an empty cherry-pick).
+    // Exclude .storkit/ so that story-file-only commits don't pass this check.
+    let diff_stat = Command::new("git")
+        .args(["diff", "--stat", "HEAD~1..HEAD", "--", ".", ":(exclude).storkit"])
+        .current_dir(project_root)
+        .output()
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .unwrap_or_default();
+
+    if diff_stat.is_empty() {
+        all_output.push_str(
+            "=== VERIFICATION FAILED: cherry-pick produced no code changes on master. ===\n",
+        );
+        cleanup_merge_workspace(project_root, &merge_wt_path, &merge_branch);
+        return Ok(SquashMergeResult {
+            success: false,
+            had_conflicts,
+            conflicts_resolved,
+            conflict_details: Some(
+                "Cherry-pick commit contains no code changes (empty diff)".to_string(),
+            ),
+            output: all_output,
+            gates_passed: true,
+        });
+    }
+
+    all_output.push_str(&format!(
+        "=== Verified: cherry-pick landed on '{base_branch}' with code changes ===\n"
+    ));
+
     // ── Clean up ──────────────────────────────────────────────────
     cleanup_merge_workspace(project_root, &merge_wt_path, &merge_branch);
     all_output.push_str("=== Merge-queue cleanup complete ===\n");