fix: address review comments

- Use optional timeoutMs parameter (undefined = use config/default)
- Extract DEFAULT_IMESSAGE_PROBE_TIMEOUT_MS to shared constants.ts
- Import constant in client.ts instead of hardcoding
- Re-export constant from probe.ts for backwards compatibility

src/imessage/constants.ts

@@ -0,0 +1,2 @@
+/** Default timeout for iMessage probe/RPC operations (10 seconds). */
+export const DEFAULT_IMESSAGE_PROBE_TIMEOUT_MS = 10_000;