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/settings.go

@@ -18,7 +18,7 @@ import "errors"
 // to presets.FromSettings separately.
 type Settings struct {
 	// Provider names the preset: "anthropic", "openai", "openrouter",
-	// "ollama" or "openai_compatible".
+	// "xai", "ollama" or "openai_compatible".
 	Provider string
 	// BaseURL is the API root. It is required for ollama and
 	// openai_compatible; the other presets fix their own, and a non-empty