feat: add Chapter 2 gold miner FSM example in Go

Port the finite state machine from "Programming Game AI by Example"
chapter 2: BaseGameEntity, a generic State[T]/StateMachine[T] pair
(Go generics standing in for the book's C++ templates), and a Miner
that cycles through digging, banking, sleeping, and drinking states.
VisitBankAndDepositGold, GoHomeAndSleepTilRested, and QuenchThirst
are original implementations based on the book's written behavior
description, since their C++ source wasn't available. Includes a
runnable demo in main.go.

go.mod

@@ -0,0 +1,3 @@
+module git.jjstudio.me/jonas/Programming-Game-AI-by-Example-Go
+
+go 1.25.5