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.

chapter-2-state-driven-agent-design/westworld/state_miner_global_state.go

@@ -0,0 +1,43 @@
+package westworld
+
+import "sync"
+
+// minerGlobalState is the Go version of the (not yet pasted)
+// singleton class MinerGlobalState from the book. Same pattern as
+// enterMineAndDigForNugget: unexported type, one shared instance,
+// reached only through the function below. This is the state whose
+// Execute runs on every single tick, regardless of whatever else
+// the miner is currently doing (see StateMachine.Update).
+type minerGlobalState struct{}
+
+var (
+	minerGlobalStateInstance *minerGlobalState
+	minerGlobalStateOnce     sync.Once
+)
+
+// MinerGlobalState is the Go stand-in for
+// "MinerGlobalState::Instance()".
+func MinerGlobalState() *minerGlobalState {
+	minerGlobalStateOnce.Do(func() {
+		minerGlobalStateInstance = &minerGlobalState{}
+	})
+	return minerGlobalStateInstance
+}
+
+var _ State[Miner] = (*minerGlobalState)(nil)
+
+// Enter, Execute and Exit are placeholders - the book hasn't shown
+// us this state's real methods yet. This stub exists only so
+// NewMiner can set it as the miner's global state and compile.
+
+func (s *minerGlobalState) Enter(m *Miner) {
+	// TODO: fill in once the book shows this method's body.
+}
+
+func (s *minerGlobalState) Execute(m *Miner) {
+	// TODO: fill in once the book shows this method's body.
+}
+
+func (s *minerGlobalState) Exit(m *Miner) {
+	// TODO: fill in once the book shows this method's body.
+}