Add TWITTER_BROWSER environment variable to allow users to control
which browser's cookies are prioritized during extraction.
Example: TWITTER_BROWSER=chrome twitter feed
Supported values: arc, chrome, edge, firefox, brave.
The specified browser is moved to the front of the extraction order.
Also adds AGENTS.md developer guide for AI coding assistants.
Co-authored-by: Agassi <413855+agassiyzh@users.noreply.github.com>
