chore: add MERGE-DEBUG traces for project_root lifecycle

Temporary diagnostic logging to track why project_root becomes None
during merge pipeline operations. Tagged with MERGE-DEBUG for easy
grep-and-remove once the root cause is confirmed fixed.

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

server/src/http/mcp.rs

@@ -1580,7 +1580,17 @@ async fn tool_merge_agent_work(args: &Value, ctx: &AppContext) -> Result<String,
         .ok_or("Missing required argument: story_id")?;
     let agent_name = args.get("agent_name").and_then(|v| v.as_str());
 
+    // TRACE:MERGE-DEBUG — remove once root cause is found
+    crate::slog!(
+        "[MERGE-DEBUG] tool_merge_agent_work called for story_id={:?}, agent_name={:?}",
+        story_id,
+        agent_name
+    );
     let project_root = ctx.agents.get_project_root(&ctx.state)?;
+    crate::slog!(
+        "[MERGE-DEBUG] tool_merge_agent_work: project_root resolved to {:?}",
+        project_root
+    );
     let report = ctx.agents.merge_agent_work(&project_root, story_id).await?;
 
     let status_msg = if report.success && report.gates_passed && report.conflicts_resolved {