That said, you presently own vested shares in the company, and that means you [likely] have minority shareholder rights. If the company does not act in your best interests as a shareholder, then you have cause for a lawsuit. So, they can't play games with your shares without opening themselves up to a lawsuit.
However, you're bailing after only six months, which implies that you believe you'll be able to create more value for yourself by doing something else rather than seeing it through with a startup. It also means that the company must create all the value while you sit back and pursue something else; there was prior work done, but how does that compare to the work that needs to be done to make your shares worth something? Your shares are essentially deadweight that disincentives the remaining teams (and investors).
A typical shareholder's agreement will specify that the company and/or other shareholders will buy back your shares for the present valuation. It is also pretty typical for such a shareholder's agreement to specify a "method valuation", like a penny/share, in case you have not yet taken outside investment.
If you do not have a shareholder's agreement that covers this scenario and you have not yet taken VC and the company still has a substantial way to go (all of which seems like your scenario, since you're asking on HN rather than acting in accordance with a contract), then the honorable thing to do is sell your shares back to the company for a token amount. You're getting off the bus; in exchange, you don't have to ride the bus, and that's worth a great deal to you.