fix(single_request): 명시된 null 템플릿 경로를 생략과 구분한다

`plan_file: null`과 `review_file: ~`은 YAML에 실제로 존재하는 키인데
mapstructure가 present nil을 생략과 같은 nil pointer로 접어버려, 승인 경계가
빈 문자열과 공백만 거부하고 null 형태는 조용히 built-in default로 떨어뜨렸다.
Viper가 돌려주는 raw preset 구조에는 두 키가 nil 값으로 남아 있으므로, strict
decode 전에 그 presence를 검사해 필드 경로와 함께 거부하고 실제로 없는 키만
fallback 신호로 남긴다.

두 필드와 두 가지 YAML null 표기를 모두 회귀로 고정하고, 기존 빈/공백, 생략,
상대 경로, clone 독립성, refresh redaction 근거는 그대로 유지한다.

Refs: agent-task/single_request_plan_review_templates/PLAN-cloud-G08.md

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/go/config/load.go

@@ -71,6 +71,9 @@ func LoadEdge(cfgFile string) (*EdgeConfig, error) {
 	}
 	if v.InConfig("execution_presets") {
 		raw := v.Get("execution_presets")
+		if err := rejectNullSingleRequestTemplatePaths(raw); err != nil {
+			return nil, err
+		}
 		var presets []ExecutionPreset
 		var metadata mapstructure.Metadata
 		decoder, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
@@ -681,6 +684,42 @@ func validateWorkspaceNumericLimits(ws WorkspaceDefinition, nodeIdx, wsIdx int)
 	return nil
 }
 
+// rejectNullSingleRequestTemplatePaths fails closed on a present-but-null
+// plan_file/review_file before the strict preset decode runs. Mapstructure
+// collapses a present raw nil into the same nil pointer as an absent key, so
+// `plan_file: null` and `review_file: ~` would otherwise be indistinguishable
+// from omission and silently select the built-in default. Only a truly absent
+// key may reach the nil-pointer fallback in resolveSingleRequestTemplates;
+// non-nil values stay with the strict decoder and loadTemplateFile, and any
+// other raw shape is left to the strict decoder's own type error.
+func rejectNullSingleRequestTemplatePaths(raw any) error {
+	presets, ok := raw.([]any)
+	if !ok {
+		return nil
+	}
+	for i, entry := range presets {
+		preset, ok := entry.(map[string]any)
+		if !ok {
+			continue
+		}
+		singleRequest, ok := preset["single_request"].(map[string]any)
+		if !ok {
+			continue
+		}
+		templates, ok := singleRequest["templates"].(map[string]any)
+		if !ok {
+			continue
+		}
+		for _, field := range []string{"plan_file", "review_file"} {
+			value, present := templates[field]
+			if present && value == nil {
+				return fmt.Errorf("execution_presets[%d] single_request.templates.%s: template path must not be empty", i, field)
+			}
+		}
+	}
+	return nil
+}
+
 // resolveSingleRequestTemplates freezes the effective Plan/Review templates for
 // every single-request preset. Only an omitted plan_file/review_file selects the
 // built-in default; every configured value is routed through loadTemplateFile so

packages/go/config/model_execution_preset_config_test.go

@@ -965,9 +965,11 @@ nodes:
 	})
 
 	// Only an omitted field selects the built-in default. An explicitly
-	// configured empty or whitespace-only path is a configuration error and
-	// must be rejected with its own field context before the loader touches
-	// the filesystem.
+	// configured empty, whitespace-only, or null path is a configuration error
+	// and must be rejected with its own field context before the loader touches
+	// the filesystem. Both YAML null spellings are covered because a present
+	// null decodes into the same nil pointer as an absent key, so the rejection
+	// has to happen on the raw preset structure before that presence is erased.
 	t.Run("explicitly configured empty template path fails closed", func(t *testing.T) {
 		cases := []struct {
 			name       string
@@ -977,8 +979,12 @@ nodes:
 		}{
 			{"empty plan_file", `""`, "", "single_request.templates.plan_file"},
 			{"whitespace plan_file", `"   "`, "", "single_request.templates.plan_file"},
+			{"null plan_file", "null", "", "single_request.templates.plan_file"},
+			{"tilde plan_file", "~", "", "single_request.templates.plan_file"},
 			{"empty review_file", "", `""`, "single_request.templates.review_file"},
 			{"whitespace review_file", "", `"   "`, "single_request.templates.review_file"},
+			{"null review_file", "", "null", "single_request.templates.review_file"},
+			{"tilde review_file", "", "~", "single_request.templates.review_file"},
 		}
 
 		for _, tc := range cases {