docs: note MODEL_NOT_IN_PLAN is verified but not in the public error table

[TheStreamCode]

What

Clarifies the \MODEL_NOT_IN_PLAN\ claim in the README after live verification against the Command Code Provider API.

Why

\MODEL_NOT_IN_PLAN\ is a real, observable error (HTTP 403, e.g. \claude-*\ models on a GOAT key), but it is not listed in the public error table on the Provider API docs, which only covers common cases. The README now states this explicitly so readers cross-checking the two sources don assume the claim is invented.

Verification

• \claude-opus-5\ on /messages\ with a GOAT key -> HTTP 403 \MODEL_NOT_IN_PLAN: ... available in Provider and above plans or extra on demand usage\
• \claude-sonnet-5\ on /messages\ -> HTTP 403 \MODEL_NOT_IN_PLAN: ... available in Pro and above plans...\
• \poolside/laguna-s-2.1-free\ -> HTTP 200 (free model, confirms auth works)
• No code/baseline changes: baseline and live endpoint are already in sync (56 = 56 models).