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/project.rs

@@ -49,6 +49,11 @@ impl ProjectApi {
     /// Close the current project and clear the stored selection.
     #[oai(path = "/project", method = "delete")]
     async fn close_project(&self) -> OpenApiResult<Json<bool>> {
+        // TRACE:MERGE-DEBUG — remove once root cause is found
+        crate::slog_error!(
+            "[MERGE-DEBUG] DELETE /project called! \
+             Backtrace: this is the only code path that clears project_root."
+        );
         fs::close_project(&self.ctx.state, self.ctx.store.as_ref()).map_err(bad_request)?;
         Ok(Json(true))
     }