Add secret missions

Randomly assign missions to the player to receive bonus points (e.g. "Receive +1 if you both cooperate").

No files, tests, or entry points are named. Start by locating the JavaScript/NodeJS code that handles players, turns, and scoring, then clarify the mission pool, assignment timing, multiplayer visibility, and bonus rules. Done should include agreed mission behavior and verification that completed missions award the intended bonus points.

Written by the indexing model from the issue text.