Vendor listing — add a provider to the catalog

A listing has two halves, and both must ship:

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.

Deep references (read before non-trivial work; do not duplicate them here):

• docs/context/guides/expanding-a-category.md — the add-a-provider playbook, verify toolbox, traps
• docs/context/architecture/catalog.md — catalog schema, cost provenance, verify pipeline, PII rules
• docs/VENDORS.md — what we told the vendor to prepare (their checklist)
• src/treg/web/vendor-listing.md — the HOSTED instructions (served at /vendor-listing) that a vendor's own coding agent follows to raise a listing PR; the dashboard's "List as vendor" modal (connections view, vendorAsk in index.html) hands vendors a prompt pointing at it. Keep the three vendor-facing surfaces (doc, hosted page, modal prompt) telling one story.

Step 0 — intake: collect the vendor facts