Vendor listing — add a provider to the catalog

A listing has three parts, and all three ship in the same PR:

1. Registry entry (src/treg/oauth_providers.py) — how a team connects a credential for the provider, and how treg verifies that credential is real.
2. Core catalog file (src/treg/catalog/<service>.yaml) — what an agent can do: 8–15 curated endpoints with capability mapping, inputs, cost + provenance, and verified examples.
3. Platform-key slot (config.py + render.yaml + fx.yaml) — so treg can serve the endpoints on its own key (tier 4). Always aim for this. A listing that is BYOK-only is the exception and needs a stated reason (no self-serve pricing, own_account data, sales-gated).

And every listing PR carries a verification evidence ledger (Step 7b). No ledger, no merge.