fix(posthog): disable feature flags, remove plugin_loaded event for billing optimization

- Remove plugin_loaded telemetry from index.ts (was 46.5% of all events, ~2.83M/month)
- Add enableLocalEvaluation: false to prevent feature flag polling/decide calls
- Add strictLocalEvaluation: true to prevent server fallback requests
- Add disableRemoteConfig: true to prevent remote config network requests
- Remove 'plugin_loaded' from PostHogActivityReason type
- Update tests: remove stale mocks, add SDK options verification test
- enableExceptionAutocapture: false already present (kept)

Estimated billing reduction: ~$960+/month from feature flag requests,
plus ~2.83M fewer events/month from plugin_loaded removal.

src/cli/cli-installer.telemetry.test.ts

@@ -39,8 +39,6 @@ describe("runCliInstaller telemetry isolation", () => {
     mock.module("../shared/posthog", () => ({
       createCliPostHog: mock(() => ({
         trackActive: mock(() => {}),
-        capture: mock(() => {}),
-        captureException: mock(() => {}),
         shutdown: mock(async () => {
           throw new Error("shutdown failed")
         }),