Fix GHES hostnames detection to use exact match for github.com

[jtquip88]

Describe the bug

Currently, any hostname ending with "github.com" (such as "slack-github.mirrors.m2d.in") is incorrectly identified as the public GitHub host github.com.

The fix changes the public github hostname check from a suffix match to an exact match:

Before: strings.HasSuffix(u.Hostname(), "github.com")
After: u.Hostname() == "github.com"

This ensures that GHES hostnames like "slack-github.mirrors.m2d.in" are correctly identified as GitHub Enterprise Server instances rather than being treated as the public github.com.
Alternative approaches:

1. I am not sure if public github hosts can have subdomains, but if needed a regex can be used to match host name patterns.
2. Add extra optional parameter like IS_GHES to explicitly indicate that this is a GHES instance.

Fixes #662

[SamMorrowDrums]

👋 We're actively looking at improving GHES support for the MCP server! If you have thoughts on GHES-specific needs, please share your feedback in this discussion. 🙏