feat(boost-greeting): implement randomized greeting messages

Introduce a collection of greeting templates to provide variety when
a user boosts the server. The system now selects a random message
from the available variants and logs the selected variant index.

src/discord/events/boostGreeting.test.ts

@@ -13,7 +13,6 @@ describe("sendBoostGreeting", () => {
     await sendBoostGreeting(channel, "user-123", "channel-456");
 
     expect(sent.length).toBe(1);
-    expect(sent[0]).toContain("Makasih udah ngeboost");
     expect(sent[0]).toContain("<@user-123>");
     expect(sent[0]).toContain("/booster-role claim");
     expect(sent[0]).toContain("di sini");