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/store/schema_test.go

@@ -364,7 +364,7 @@ func TestSchemaRotationReviewEventAllowed(t *testing.T) {
 // both ai_settings and ai_runs.
 func TestSchemaAIProvidersAllowed(t *testing.T) {
 	s := seededStore(t)
-	for _, provider := range []string{"openai_compatible", "anthropic", "ollama", "openai", "openrouter"} {
+	for _, provider := range []string{"openai_compatible", "anthropic", "ollama", "openai", "openrouter", "xai"} {
 		for _, table := range []string{"ai_settings", "ai_runs"} {
 			inRolledBackTx(t, s, func(tx *sql.Tx) {
 				row := testRow(table, 1)