refactor(models): remove responses API model registration hack

Fantasy v0.21.0 natively includes gpt-5.5 and other newer models in
its responsesModelIDs/responsesReasoningModelIDs lists, making our
workaround unnecessary.

- Delete responses_models.go (go:linkname hack + RegisterResponsesModels)
- Delete responses_models_test.go
- Replace isResponsesAPIModel/isResponsesReasoningModel heuristics with
  direct openai.IsResponsesModel/openai.IsResponsesReasoningModel calls
- Remove RegisterResponsesModels calls from registry init/reload
- Remove hack documentation from AGENTS.md
- Update all deps (fantasy v0.21.0, smithy-go, ultraviolet, etc.)

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 v0.20.0
+	charm.land/fantasy v0.21.0
 	charm.land/huh/v2 v2.0.3
 	charm.land/lipgloss/v2 v2.0.3
 	github.com/alecthomas/chroma/v2 v2.23.1
@@ -14,7 +14,7 @@ require (
 	github.com/charmbracelet/fang v1.0.0
 	github.com/charmbracelet/log v1.0.0
 	github.com/charmbracelet/openai-go v0.0.0-20260319145158-d0740cc34266
-	github.com/charmbracelet/ultraviolet v0.0.0-20260420095748-421e4a7fa8d7
+	github.com/charmbracelet/ultraviolet v0.0.0-20260422141423-a0f1f21775f7
 	github.com/charmbracelet/x/editor v0.2.0
 	github.com/clipperhouse/displaywidth v0.11.0
 	github.com/clipperhouse/uax29/v2 v2.7.0
@@ -51,7 +51,7 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/sso v1.30.16 // indirect
 	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.35.20 // indirect
 	github.com/aws/aws-sdk-go-v2/service/sts v1.42.0 // indirect
-	github.com/aws/smithy-go v1.25.0 // indirect
+	github.com/aws/smithy-go v1.25.1 // indirect
 	github.com/catppuccin/go v0.3.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/charmbracelet/anthropic-sdk-go v0.0.0-20260223140439-63879b0b8dab // indirect
@@ -59,9 +59,9 @@ require (
 	github.com/charmbracelet/harmonica v0.2.0 // indirect
 	github.com/charmbracelet/lipgloss v1.1.1-0.20250404203927-76690c660834 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect
-	github.com/charmbracelet/x/exp/charmtone v0.0.0-20260420102150-fe550f2efce5 // indirect
+	github.com/charmbracelet/x/exp/charmtone v0.0.0-20260426004601-d5e63ff0b9ca // indirect
 	github.com/charmbracelet/x/exp/ordered v0.1.0 // indirect
-	github.com/charmbracelet/x/exp/slice v0.0.0-20260420102150-fe550f2efce5 // indirect
+	github.com/charmbracelet/x/exp/slice v0.0.0-20260426004601-d5e63ff0b9ca // indirect
 	github.com/charmbracelet/x/exp/strings v0.1.0 // indirect
 	github.com/charmbracelet/x/json v0.2.0 // indirect
 	github.com/charmbracelet/x/termios v0.1.1 // indirect
@@ -82,10 +82,10 @@ require (
 	github.com/googleapis/enterprise-certificate-proxy v0.3.15 // indirect
 	github.com/googleapis/gax-go/v2 v2.22.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect
-	github.com/kaptinlin/go-i18n v0.4.2 // indirect
-	github.com/kaptinlin/jsonpointer v0.4.18 // indirect
-	github.com/kaptinlin/jsonschema v0.7.8 // indirect
-	github.com/kaptinlin/messageformat-go v0.5.2 // indirect
+	github.com/kaptinlin/go-i18n v0.4.4 // indirect
+	github.com/kaptinlin/jsonpointer v0.4.19 // indirect
+	github.com/kaptinlin/jsonschema v0.7.11 // indirect
+	github.com/kaptinlin/messageformat-go v0.6.0 // indirect
 	github.com/mitchellh/hashstructure/v2 v2.0.2 // indirect
 	github.com/muesli/mango v0.2.0 // indirect
 	github.com/muesli/mango-cobra v1.3.0 // indirect
@@ -129,7 +129,7 @@ require (
 	github.com/charmbracelet/x/term v0.2.2 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/lucasb-eyer/go-colorful v1.4.0 // indirect
-	github.com/mattn/go-isatty v0.0.21 // indirect
+	github.com/mattn/go-isatty v0.0.22 // indirect
 	github.com/mattn/go-runewidth v0.0.23 // indirect
 	github.com/muesli/cancelreader v0.2.2 // indirect
 	github.com/muesli/termenv v0.16.0 // indirect