watch to watch the dir for changes and trigger a rebuild automagically-- flag is that it is an npm specifc flag indicating the end of options for npm specific commands, such that in the case you mentioned that yarn dev becomes a command line option for the wrangler pages dev command. In that case, my next step would probably be to figure out if there is a flag on this page that does what you are looking for: https://developers.cloudflare.com/workers/wrangler/commands/#devwatch since that is typically how these things work anyways, but that is where I would look if I better understood the specifics of your needs.
yarn dev would output anywayworkerd, with all of the bells and whistles that a Worker would havewrangler.toml/jsonc. https://developers.cloudflare.com/workers/configuration/routing/workers-dev/#disabling-workersdevWe plan to bridge the gaps between Workers and Pages and provide ways to migrate your Pages projects to Workers.