fix(424): add throttled field to all StoryAgent ctors and handle HardBlock in ws.rs

The initial commit added the `throttled` field to `StoryAgent` but missed
several construction sites in lifecycle.rs, test_helpers.rs, and scan.rs.
Also adds the `HardBlock` match arm in the WebSocket event conversion and
minor CSS/import ordering fixes.

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

server/src/agents/pool/auto_assign/scan.rs

@@ -154,6 +154,7 @@ mod tests {
             project_root: None,
             log_session_id: None,
             merge_failure_reported: false,
+            throttled: false,
         }
     }