Prevent AlphaClaw from starting on port 18789

[emileferreira]

This resolves #26, by preventing AlphaClaw from starting on port 18789. An alternative approach would be to change the port used for OpenClaw when AlphaClaw uses this port.

Changes Made:

1. Updated bin/alphaclaw.js:
   • Improved flag parsing to correctly recognise --port and --root-dir even when placed after the command (e.g., alphaclaw start --port 18789).
   • Added a fatal error check that prevents AlphaClaw from starting if the resolved port is 18789 (the port reserved for the OpenClaw gateway).
2. Added unit tests:
   • Created tests/bin/alphaclaw.test.js to verify the port check behavior.
   • Verified that PORT=18789 environment variable triggers the error.
   • Verified that --port 18789 CLI flag triggers the error.
   • Verified that other ports allow the process to continue (until the next check, such as SETUP_PASSWORD).