Introduce ug command and Phase 1 migration path

[rohita5l]

Summary

• add the ug console entry point while retaining ucode as a supported alias
• make the README and CLI commands/guidance lead with ug, while preserving Unity Gateway branding in agent launch banners and Smart Router notices
• make ug upgrade distribution-aware: upgrade ucode normally before cutover, migrate only on uv's confirmed metadata-name mismatch, then upgrade unity-gateway normally afterward
• preserve the existing installation on unrelated upgrade failures and verify both ug and ucode after every successful upgrade
• add focused coverage for both entry points, invocation-aware help, all three upgrade lifecycle stages, failure safety, renamed guidance, and launch branding

Testing

• .venv/bin/pytest tests/test_cli.py tests/test_entry_points.py -q (284 passed)
• .venv/bin/ruff check .
• .venv/bin/ruff format --check src/ucode/cli.py tests/test_cli.py
• .venv/bin/ty check src/ucode/cli.py
• git diff --check
• isolated real-uv lifecycle probe: confirmed ucode metadata mismatch at cutover, successful reinstall as unity-gateway, and normal post-cutover uv tool upgrade unity-gateway

Release requirement

Merge and tag this commit while [project].name is still ucode. That immutable tag is the bridge release and must exist before the distribution-name cutover.