fix(models): auto-register new OpenAI models for Responses API routing

Fantasy's hardcoded responsesModelIDs list gates whether a model uses
the Responses API or Chat Completions code path. When a new model
(e.g. gpt-5.5) is added via `kit update-models` but fantasy hasn't
been updated yet, the type mismatch between *ResponsesProviderOptions
and *ProviderOptions causes a crash.

- Add isResponsesAPIModel()/isResponsesReasoningModel() helpers that
  supplement fantasy's checks with prefix-based heuristics for modern
  OpenAI model families (gpt-4.1+, gpt-5+, o-series, codex, chatgpt)
- Add RegisterResponsesModels() using go:linkname to append missing
  model IDs from our database into fantasy's internal slices at init
  time and after ReloadGlobalRegistry()
- Replace all direct openai.IsResponsesModel/IsResponsesReasoningModel
  calls in providers.go with the new helpers
- Merge embedded + cached model databases instead of cache-only fallback
- Bump fantasy v0.19.0 -> v0.20.0 to match existing import usage
- Document the technique and model-family update process in AGENTS.md

go.mod

@@ -5,7 +5,7 @@ go 1.26.2
 require (
 	charm.land/bubbles/v2 v2.1.0
 	charm.land/bubbletea/v2 v2.0.6
-	charm.land/fantasy v0.19.0
+	charm.land/fantasy v0.20.0
 	charm.land/huh/v2 v2.0.3
 	charm.land/lipgloss/v2 v2.0.3
 	github.com/alecthomas/chroma/v2 v2.23.1