Support configurable Git providers and credentials for self-hosted Git servers

What variant of Codex are you using?

App

What feature would you like to see?

Codex should support alternate Git platforms through a configurable Git provider section in settings:

Settings -> Coding -> Git

Users should be able to add Git hosts beyond github.com, including self-hosted instances, and provide the
credentials Codex needs to resolve repository, branch, issue, pull request, and merge request context.

Examples:

• GitLab.com
• self-hosted GitLab CE / EE
• Forgejo
• Gitea
• other Git-compatible hosts where API support can be added over time

Additional information

There are existing requests for this in related areas:

• #8156 requested broader support for platforms other than GitHub, including GitLab, Gitea, Gogs, and Forgejo.
• #26963 asks for GitLab Merge Request links in the Codex App sidebar, including support for self-hosted GitLab.

Codex currently has first-class GitHub workflow awareness, but many teams use self-hosted or non-GitHub Git platforms.