fix(plugin): scope synthetic-idle dedup bypass to matching marker

Only clear recentAnyIdles when the stored marker matches the synthetic idle timestamp for the same session, preventing accidental clobbering of newer idle markers. Add a regression test to verify other sessions keep their dedup state during this bypass path.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/event.ts

@@ -417,7 +417,10 @@ export function createEventHandler(args: {
           recentSyntheticIdles.delete(sessionID);
           // Let real idle events through even when a synthetic idle fired moments earlier.
           // OpenCode diagnostics expect a concrete session.idle event signal.
-          recentAnyIdles.delete(sessionID);
+          const lastAnyIdleAt = recentAnyIdles.get(sessionID);
+          if (lastAnyIdleAt === emittedAt) {
+            recentAnyIdles.delete(sessionID);
+          }
         }
         recentRealIdles.set(sessionID, now);
         if (!shouldDispatchIdleEvent(sessionID, now)) {