fix(nlab-api): fetch target branches outside configured refspecs

[JacobZyy]

Repositories cloned with --single-branch could report an existing target branch as missing because fetching origin honored the restricted fetch refspec. Creating a tracking branch also failed when the requested branch was outside that refspec.

Fetch the requested branch into its explicit remote-tracking ref and create the local branch with --no-track. Existing explicit fast-forward pulls continue to update it. Preserve the repository's fetch configuration and existing worktree protections. The shared repository path covers initialization, generation, and Discover.

Validation:

• Reproduced the original failure with a single-branch clone regression, then verified branch creation, subsequent updates, unchanged fetch configuration, and failure without switching when a branch is absent.
• Rust formatting, Clippy with warnings denied, and all 389 Rust tests passed.
• Documentation build passed.