Add xAI Grok provider preset

Preset xai builds the OpenAI-compatible adapter against the fixed base URL
https://api.x.ai/v1 with a required API key and no extra headers. It
defaults to json_schema structured output and lets the household select
json_object as a fallback; a request is never retried in another mode.

Migration 0005_ai_provider_xai rebuilds ai_settings and ai_runs the 0004
way so their provider CHECK also accepts xai, keeping every 0004 value.

Maestro task d8be638f-297f-4b7a-a545-74d193055904.

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

internal/ai/llm/presets/presets.go

@@ -145,6 +145,15 @@ var all = map[string]Preset{
 		StructuredOutputs:        []openai.StructuredOutput{openai.JSONSchema, openai.JSONObject},
 		OmitTemperature:          []*regexp.Regexp{openRouterReasoningModels},
 	},
+	"xai": {
+		Name:                     "xai",
+		Adapter:                  AdapterOpenAI,
+		BaseURL:                  "https://api.x.ai/v1",
+		Key:                      KeyRequired,
+		StructuredOutput:         openai.JSONSchema,
+		FallbackStructuredOutput: openai.JSONObject,
+		StructuredOutputs:        []openai.StructuredOutput{openai.JSONSchema, openai.JSONObject},
+	},
 	"ollama": {
 		Name:    "ollama",
 		Adapter: AdapterOllama,