refactor: unify SDK model factories into internal/models

Move CreateModel, BuildReasoningOptions, ReasoningBudgetTokens and
related types from internal/agent to internal/models as NewSDKChatModel,
SDKModelConfig, etc. This eliminates duplicate ClientType constants and
centralises all Twilight AI SDK instance creation in a single package.

NewSDKEmbeddingModel now accepts a clientType parameter and dispatches
to the native Google embedding provider for google-generative-ai,
instead of always using the OpenAI-compatible endpoint.

internal/memory/adapters/builtin/dense_runtime.go

@@ -35,6 +35,7 @@ type denseRuntime struct {
 
 type denseModelSpec struct {
 	modelID    string
+	clientType string
 	baseURL    string
 	apiKey     string
 	dimensions int
@@ -74,7 +75,7 @@ func newDenseRuntime(providerConfig map[string]any, queries *dbsqlc.Queries, cfg
 		return nil, fmt.Errorf("dense runtime: %w", err)
 	}
 
-	embedModel := models.NewSDKEmbeddingModel(spec.baseURL, spec.apiKey, spec.modelID, denseEmbedTimeout)
+	embedModel := models.NewSDKEmbeddingModel(spec.clientType, spec.baseURL, spec.apiKey, spec.modelID, denseEmbedTimeout)
 
 	return &denseRuntime{
 		qdrant:     qClient,
@@ -565,6 +566,7 @@ func resolveDenseEmbeddingModel(ctx context.Context, queries *dbsqlc.Queries, mo
 	}
 	return denseModelSpec{
 		modelID:    strings.TrimSpace(row.ModelID),
+		clientType: strings.TrimSpace(provider.ClientType),
 		baseURL:    strings.TrimSpace(provider.BaseUrl),
 		apiKey:     strings.TrimSpace(provider.ApiKey),
 		dimensions: *cfg.Dimensions,

internal/memory/memllm/client.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/memohai/memoh/internal/agent"
 	adapters "github.com/memohai/memoh/internal/memory/adapters"
+	"github.com/memohai/memoh/internal/models"
 )
 
 const (
@@ -42,7 +43,7 @@ func New(cfg Config) *Client {
 }
 
 func (c *Client) model() *sdk.Model {
-	return agent.CreateModel(agent.ModelConfig{
+	return models.NewSDKChatModel(models.SDKModelConfig{
 		ModelID:    c.cfg.ModelID,
 		ClientType: c.cfg.ClientType,
 		APIKey:     c.cfg.APIKey,