Discover claimable startup slugs

Clerk session or agt_ token with startup:edit (live tier re-checked) - the SAME gate the claim route uses, so anyone who can file a claim can discover slugs and listing never exposes more than the claim flow already assumes. bound:false means unbound and therefore claimable; bound:true means the slug already has a founder and a claim would be a no-op an operator can never approve. This closes the dead end where the claim required a slug that nothing agent-reachable could enumerate.