feat: provider modules — ProviderProfile ABC, 29 providers, fetch_models, transport single-path
Introduces providers/ as the single source of truth for every inference
provider. All 29 providers declared with correct data cross-checked against
auth.py, runtime_provider.py and auxiliary_client.py.
Rebased onto main (30307a980). Incorporates post-salvage fixes from
56724147e (gmi aux model google/gemini-3.1-flash-lite-preview, already set in providers/gmi.py).
setup_model_provider() now delegates to select_provider_and_model()
from main.py, so new providers only need to be wired in main.py.
Removed setup.py from file checklists, replaced the setup.py section
with a tip explaining the automatic inheritance.
The install script creates venv/ but several docs referenced .venv/,
causing agents to fail with 'No such file or directory' when following
AGENTS.md instructions.
Fixes#2066
