fix(nlab-api): update backend before generate

[JacobZyy]

Summary

• fast-forward the configured backend branch from origin before every generation
• refresh the repository commit after pulling, then synchronize CodeGraph
• fail on branch conflicts, divergence, network errors, or dirty tracked files reported by Git
• verify untracked .codegraph state does not block updates

Verification

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked