Remove demo combatants and allow empty encounters

Empty encounters are now valid (INV-1 updated). New sessions start
with zero combatants instead of pre-populated Aria/Brak/Cael.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/domain/src/__tests__/advance-turn.test.ts

@@ -169,9 +169,9 @@ describe("advanceTurn", () => {
 	});
 
 	describe("invariants", () => {
-		it("INV-1: createEncounter rejects empty combatant list", () => {
+		it("INV-1: createEncounter accepts empty combatant list", () => {
 			const result = createEncounter([]);
-			expect(isDomainError(result)).toBe(true);
+			expect(isDomainError(result)).toBe(false);
 		});
 
 		it("INV-2: activeIndex always in bounds across all scenarios", () => {