Fix test isolation to prevent loading real user config

Tests were picking up real ~/.warelay/warelay.json with emojis and
prefixes (like "🦞"), causing test assertions to fail. Added proper
config mocks to all test files.

Changes:
- Mock loadConfig() in index.core.test.ts, inbound.media.test.ts,
  monitor-inbox.test.ts
- Update test-helpers.ts default mock to disable all prefixes
- Tests now use clean config: no messagePrefix, no responsePrefix,
  no timestamp, allowFrom=["*"]

This ensures tests validate core behavior without user-specific config.
The responsePrefix feature itself is already fully config-driven - this
only fixes test isolation.

src/index.core.test.ts

@@ -9,6 +9,18 @@ import { createMockTwilio } from "../test/mocks/twilio.js";
 import * as exec from "./process/exec.js";
 import { withWhatsAppPrefix } from "./utils.js";
 
+// Mock config to avoid loading real user config
+vi.mock("../src/config/config.js", () => ({
+  loadConfig: vi.fn().mockReturnValue({
+    inbound: {
+      allowFrom: ["*"],
+      messagePrefix: undefined,
+      responsePrefix: undefined,
+      timestampPrefix: false,
+    },
+  }),
+}));
+
 // Twilio mock factory shared across tests
 vi.mock("twilio", () => {
   const { factory } = createMockTwilio();