Add gpt-6.1-sol, gpt-6-sol and gpt-6-luna to the provider capability
catalog and make gpt-6.1-sol/medium the codex default (needs codex CLI
>= 0.159.1; verified live on 0.160). Drop ultra from gpt-5.6-luna, which
the Codex model metadata never listed, and note the gpt-5.5 retirement
on 2026-10-14.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>