fix(ai): recognise the gpt-6 model family in every model gate

The ChatGPT Codex backend and the OpenAI API serve gpt-6-sol, gpt-6-luna,
gpt-6-astra and gpt-6.1-sol, but the reasoning, Chat Completions and vision
gates only knew the gpt-5 line. A gpt-6 model got no reasoning control and
no real "off", went out on the legacy wire with max_tokens and a
temperature that reasoning models refuse, and was denied image input.

The three gates now read one classifier, openai-model-family.ts, so a new
generation is taught in one place. Every GPT from 5.1 on, the gpt-6 line
included, is the family that takes effort "none"; every GPT from 5 on reads
images. The upstream catalog (codex-rs/models-manager/models.json) lists
image input for every gpt-6 slug and the same reasoning presets as the
gpt-5.6 line; the presets omit "none" for gpt-5.5 too, which accepts it.

The Codex slug ladder moves into codex-slug-chain.ts and gains gpt-6-sol,
gpt-6.1-sol and gpt-6-luna. gpt-5.6-terra stays first as the slug verified
against the backend; the catalog names gpt-6-sol as its upgrade, and a
rotation retires a whole line at once, so the gpt-6 rungs follow before
terra's own siblings. The ladder test now holds every rung to a recognised
reasoning family with a real off and image input. The OpenAI presets offer
gpt-6.1-sol and gpt-6-luna.

src/components/settings/ai/shared.ts

@@ -99,10 +99,13 @@ export const DEFAULT_CHAIN: readonly ChainEntry[] = [
  * OpenAI model presets — the wire-up keeps users out of the freetext
  * box for the common case. The `__custom__` sentinel surfaces a
  * freetext input so power users can target preview models or
- * fine-tunes (e.g. `gpt-5`, which is admitted via the env override on
- * the Codex backend but not yet a documented OpenAI model id).
+ * fine-tunes. The gpt-6 entries are the current workhorse and its
+ * lighter sibling; every preset is a family `openai-model-family.ts`
+ * recognises, so the reasoning, token-field and vision gates fit it.
  */
 export const OPENAI_MODEL_PRESETS = [
+  "gpt-6.1-sol",
+  "gpt-6-luna",
   "gpt-4o",
   "gpt-4o-mini",
   "gpt-4-turbo",