fix(publisher): support --file flag in publish command

[majiayu000]

Summary

• Added proper flag parsing using Go's flag package
• mcp-publisher publish --file path/to/server.json now works correctly
• Maintains backward compatibility with positional arguments
• Improved error messages to show actual file path when custom file not found

Test

# Before: Error: server.json not found
mcp-publisher publish --file .registry/server.json

# After: reads .registry/server.json correctly

Fixes #767

[rdimitrov]

Hey, @majiayu000, unfortunately there's already another PR filed for addressing the same issue prior this one. I'll close this one as the proposed implementation is quite similar to it and it's fair to credit the other one being first.

Feel free to comment there in case you find something it missed although I think we'll probably try to address this by using cobra 👍

Thanks in advance! 🙏

[majiayu000]

Thanks for the review!