Turn off posthog

[kaovilai]

Ability to turn off posthog.

https://github.com/browseros-ai/BrowserOS-agent/blob/af671ad780c90cbe20eb22630fdc44f778c27dbc/src/background/index.ts#L19-L26

[shadowfax92]

Will add a flag in chrome://settings/browseros-settings to disable this.