Add a “summoned” bitfield to struct monst, which is used to identify monsters that have been created through magical means. Should be a bitfield rather than a flag so that it can track different details of how it was summoned (from trap/wand/etc, by player or by something else; it probably doesn’t warrant a mextra struct). Summoned monsters can have various anti-farming nerfs applied to them: no training skills, no corpses, no deathdrops, no starting inventory, no experience, etc. Possibly, summoned monsters also disappear after a few hundred turns.