Fix port mapping behavior in VSCode extension

When users cleared port mappings in Advanced Options, default ports
were still being applied. Now tracks whether user modified the ports
field and only applies defaults if unmodified.

- Removed auto-fill of ports field to avoid confusion
- Track portsModified flag via input event listener
- Apply template default ports only if user hasn't touched the field
- Respect explicit clearing of ports (user wants no mappings)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: andystime