Enhancement: Support GitHub Enterprise by using URL API for PR parsing

## Summary

Currently, the GitHubService uses a hardcoded regex pattern that only supports github.com URLs:

```typescript
const match = pr.url.match(/github\.com\/([^/]+)\/([^/]+)\/pull\/(\d+)/);
```

## Proposed Enhancement

Replace the regex with the URL API to support any host, including GitHub Enterprise:

```typescript
const u = new URL(pr.url);
const parts = u.pathname.split('/').filter(Boolean);
const [owner, repo, , prNumberStr] = parts;
```

## Context

This enhancement was suggested in PR #2 but deferred for future consideration as GitHub Enterprise support is not currently required for the project.

## References

- Original PR: https://github.com/angelxmoreno/mcp-github-code-review/pull/2
- Discussion: https://github.com/angelxmoreno/mcp-github-code-review/pull/2#discussion_r2308170739

## Priority

Low - only needed if GitHub Enterprise support becomes a requirement.