Prepare repository references for Unity Gateway rename

[rohita5l]

Summary

• point ug upgrade and Doctor reinstalls at the future databricks/unity-gateway repository
• update install, clone, and contributor documentation for the renamed repository
• update focused upgrade tests while keeping the distribution and internal package named ucode until Phase 3

Merge order

Do not merge before the GitHub repository is renamed. The new URL is intentionally not live yet.

1. Rename databricks/ucode to databricks/unity-gateway.
2. Verify the old /ucode clone and git-install URLs redirect to the new repository.
3. Mark this PR ready and merge it.

Testing

• uv run pytest tests/test_cli.py tests/test_doctor.py tests/test_entry_points.py -q (332 passed)
• uv run ruff check AGENTS.md README.md src/ucode/cli.py src/ucode/doctor.py tests/test_cli.py tests/test_doctor.py
• uv run ruff format --check src/ucode/cli.py src/ucode/doctor.py tests/test_cli.py tests/test_doctor.py
• git diff --check