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/index.telemetry.test.ts

@@ -30,14 +30,6 @@ const mockCreateHooks = mock(() => ({
   claudeCodeHooks: undefined,
 }))
 const mockCreatePluginInterface = mock(() => ({}))
-const mockCreatePluginPostHog = mock(() => ({
-  trackActive: () => {
-    throw new Error("telemetry failed")
-  },
-  shutdown: mock(async () => {}),
-}))
-const mockGetPostHogDistinctId = mock(() => "plugin-distinct-id")
-
 function installModuleMocks(): void {
   mock.module("./cli/config-manager/config-context", () => ({
     initConfigContext: mockInitConfigContext,
@@ -98,10 +90,6 @@ function installModuleMocks(): void {
       cleanupTempDirectoryClients: mock(async () => {}),
     },
   }))
-  mock.module("./shared/posthog", () => ({
-    createPluginPostHog: mockCreatePluginPostHog,
-    getPostHogDistinctId: mockGetPostHogDistinctId,
-  }))
 }
 
 describe("oh-my-openagent telemetry isolation", () => {