Adds is_distribution_available(), _require_companion(), create_manager(),
set_active_manager(), clear_active_manager(), notify_workers_to_start(),
stop_all_workers(), get_master_url() plus INSTALL_INSTRUCTIONS.
create_manager passes three callbacks through to the companion's
DistributionManager (job_completed_check required, build_prompt_with_triggers
and get_model_cache_key optional) — avoids bidirectional imports back into
main USCG that the original distribution_manager.py had at line 91 (and 419).
Lazy try-import of the companion returns a clear error if not installed.
Defensive minimum-version check (>= 0.1.0). clear_active_manager is silent
when companion missing so cleanup paths are safe even if companion gets
uninstalled mid-run.
Also updates tests/conftest.py to importlib-load the real distribution.py
so facade tests exercise actual code (matches Phase 1's remote_vae pattern).
Old code paths in generation_orchestrator/distribution_routes/etc still
active; the swap lands in the next commits.
Jason Hoku
13149ff5f9