fix(plugin): normalize event session ids

Handle OpenCode session events that carry the session ID under properties.info.id or properties.info.sessionID so background tasks and continuation hooks do not miss idle/error/delete events.

Add regression coverage for nested session.idle events completing background tasks and waking continuation hooks.

src/hooks/session-notification.test.ts

@@ -375,6 +375,47 @@ describe("session-notification", () => {
     expect(notificationCalls).toHaveLength(0)
   })
 
+  test("should mark session activity on message.part.updated event with part session id", async () => {
+    // given - main session is set
+    const mainSessionID = "main-part-activity"
+    setMainSession(mainSessionID)
+
+    const hook = createSessionNotification(createMockPluginInput(), {
+      idleConfirmationDelay: 50,
+      skipIfIncompleteTodos: false,
+      activityGracePeriodMs: 0,
+    })
+
+    // when - session goes idle, then streamed assistant activity fires
+    await hook({
+      event: {
+        type: "session.idle",
+        properties: { sessionID: mainSessionID },
+      },
+    })
+
+    await hook({
+      event: {
+        type: "message.part.updated",
+        properties: {
+          part: {
+            id: "part-1",
+            messageID: "msg-1",
+            sessionID: mainSessionID,
+            type: "text",
+            text: "still working",
+          },
+        },
+      },
+    })
+
+    // Wait for idle delay to pass
+    await new Promise((resolve) => setTimeout(resolve, 100))
+
+    // then - notification should NOT be sent (streaming activity cancelled it)
+    expect(notificationCalls).toHaveLength(0)
+  })
+
   test("should mark session activity on tool.execute.before event", async () => {
     // given - main session is set
     const mainSessionID = "main-tool"