[MafiaGame] Add Traitor's Gambit game mode.

[AfterWorld]

Implements a new paranoia-inducing game mode where multiple hidden Town Traitors work for the Mafia without knowing each other's identities.

Key Features:

• Variable traitor count (2-6) based on player count
• Traitors are hidden from Mafia and each other
• Any Villager role can be a traitor (Detective, Doctor, Mayor, etc.)
• Scales dynamically with player count
• Creates extreme paranoia - even confirmed roles cannot be trusted

Changes:

• modes.py: Add TraitorsGambit mode class with role distribution logic
• game.py: Update traitor assignment to support multiple traitors
• game.py: Add special notifications for Traitor's Gambit mode"