Add GitHub Enterprise Server support

[nissessenap]

Summary

Add support for GitHub Enterprise Server (GHES) by allowing users to configure a custom GitHub API base URL.

Background

The GithubAPIURL option was removed during the ghinstallation refactor (PR #32). While the previous implementation wasn't properly wired up, the option should be restored and correctly integrated for users running GHES.

What needs to change

API server (pkg/api/)

• Add GithubAPIURL back to Options in server.go
• Add GithubAPIURL CLI flag/env var back to APICmd in cmd/shepherd/api.go
• In NewGitHubClient(), set AppsTransport.BaseURL when a custom URL is provided
• ghinstallation supports this via ghinstallation.NewAppsTransportWithOptions() or setting atr.BaseURL after creation

GitHub adapter (pkg/adapters/github/)

• Add GithubAPIURL to adapter Options
• Pass it through to the GitHub client constructor
• The adapter's Client already uses go-github's WithEnterpriseURLs() for the comment-posting client

CLI (cmd/shepherd/main.go)

• Add GithubAPIURL flag with default:"https://api.github.com" and env:"SHEPHERD_GITHUB_API_URL" to both APICmd and GitHubCmd

Acceptance criteria

• Both API server and GitHub adapter accept a --github-api-url / SHEPHERD_GITHUB_API_URL flag
• When set, ghinstallation targets the custom URL for token generation
• When set, the adapter's GitHub client targets the custom URL for comments
• Default behavior (public GitHub) remains unchanged when the option is empty or set to https://api.github.com
• Tests cover custom URL wiring