fix(424): correct WatcherEvent::HardBlock to RateLimitHardBlock in pool/mod.rs

Story 424's merge used the wrong variant name HardBlock instead of
RateLimitHardBlock, breaking master compilation.

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

server/src/agents/pool/mod.rs

@@ -65,7 +65,7 @@ impl AgentPool {
                     };
                     let (story_id, agent_name) = match &event {
                         WatcherEvent::RateLimitWarning { story_id, agent_name }
-                        | WatcherEvent::HardBlock { story_id, agent_name, .. } => {
+                        | WatcherEvent::RateLimitHardBlock { story_id, agent_name, .. } => {
                             (story_id.clone(), agent_name.clone())
                         }
                         _ => continue,