fix(imessage): unify timeout configuration with configurable probeTimeoutMs

- Add probeTimeoutMs config option to channels.imessage
- Export DEFAULT_IMESSAGE_PROBE_TIMEOUT_MS constant (10s) from probe.ts
- Propagate timeout config through all iMessage probe/RPC operations
- Fix hardcoded 2000ms timeouts that were too short for SSH connections

Closes: timeout issues when using SSH wrapper scripts (imsg-ssh)

src/imessage/client.ts

@@ -149,6 +149,7 @@ export class IMessageRpcClient {
       params: params ?? {},
     };
     const line = `${JSON.stringify(payload)}\n`;
+    // Default timeout matches DEFAULT_IMESSAGE_PROBE_TIMEOUT_MS from probe.ts
     const timeoutMs = opts?.timeoutMs ?? 10_000;
 
     const response = new Promise<T>((resolve, reject) => {