Search accessible GitHub repositories when adding a project, including organization repos

[jakintosh]

Problem

When adding a project from GitHub, the repository field accepts only an exact owner/name or URL, and a bare name resolves under my personal account. To clone a repository from an organization I belong to, I have to leave T3 Code, look up its exact name, and paste it back.

This was previously raised in #8654, which was closed with a request to bring it here and cover access checks, pagination, and the manual fallback.

Proposed scope

For a signed-in GitHub provider, typing in the repository field searches the repositories the account can access and lets me pick one.

• Access: results include the personal, collaborator, and organization repositories the signed-in account can read, and private repositories appear only when the account has access.
• Pagination: works for accounts with many repositories without loading them all up front.
• Fallback: typing or pasting owner/name or a URL keeps working as it does now.

Environment

T3 Code 0.1.7, GitHub signed in through gh.