fix(telemetry): dedupe plugin_loaded event to once per UTC day

The plugin_loaded event was emitted on every plugin reload, generating
high event volume on PostHog (proportional to opencode restarts per
user per day). With MAU > 60K and active power users restarting
frequently, this drove unnecessary event spend.

Add a separate daily dedup state field (lastPluginLoadedDayUTC) so the
plugin_loaded capture only fires once per UTC day per machine. The
existing daily activity dedup (lastActiveDayUTC, used by
omo_daily_active) is preserved as an independent gate so the two
dimensions cannot overwrite each other in the activity state file.

src/index.telemetry.test.ts

@@ -104,6 +104,12 @@ function installModuleMocks(): void {
     createPluginPostHog: mockCreatePluginPostHog,
     getPostHogDistinctId: mockGetPostHogDistinctId,
   }))
+  mock.module("./shared/posthog-activity-state", () => ({
+    getPluginLoadedCaptureState: () => ({
+      dayUTC: "2026-04-18",
+      capturePluginLoaded: true,
+    }),
+  }))
 }
 
 describe("oh-my-openagent telemetry isolation", () => {