Let an environment choose the default GitHub owner for bare repository names

[Mihai-TanaCorp]

First, thank you Theo and the whole team for T3 Code. It is a wonderful tool, and it has changed my life.

All our repositories live under the GitHub organization TanaCorpGroup, while the account signed in to T3 Code is Mihai-TanaCorp. When we add or clone a project by its bare name (for example rb.website), T3 Code 0.1.7 resolves it to the viewer login, Mihai-TanaCorp/rb.website, and GitHub returns 404. The repository is TanaCorpGroup/rb.website.

This happens for every bare name on every machine we use: the macOS desktop app and several Linux remote environments. Typing TanaCorpGroup/<name> or pasting the URL works, but the organization has to be repeated for every clone.

The error message itself is already addressed by the open #31239 (show the resolved owner/name on not-found), which we support. This proposal is only about which owner is used for bare names.

Proposal: an optional, per-environment default owner for bare-name clone lookups on GitHub.

• Precedence: a saved setting in Settings → Source control → GitHub, then a server launch environment variable (for example T3CODE_GITHUB_DEFAULT_REPOSITORY_OWNER, useful for headless remote servers), then the viewer login as today.
• With nothing configured, behavior is unchanged. Explicit owner/name and full URLs always win.
• Out of scope: repository browsing, organization discovery, retrying across owners, account switching, repository creation, other providers.

We have a small implementation ready (GitHub provider, settings schema and form, tests, user guide). Would this fit the "focused configuration option for an established capability" route in CONTRIBUTING.md? If the direction or the mechanism should be different (per-host default, settings only, no environment variable), we are happy to adapt it.