✨ feat(spx): issue marker 支持第三种类型 prototype

- --type 三选一 spec|plan|prototype，marker 行写 <!-- spx:prototype=路径 -->，
  与 spec/plan 共用同一套增量更新逻辑；类型→state 字段的映射收成一张表
- 同步写 state JSON 的 prototypeFile（pattern 要求 dir/.../x.json）
- 顺手补上 b62e58d 漏掉的 prototypeSessionId / prototypeProfilePath：
  schema 与 knownStateFields 都没有它们，带原型会话的工单一 merge
  就会被 additionalProperties 拒掉

Claude-Session: https://claude.ai/code/session_01Kgmb1LvrPmFX3NoUDB4zZC

vscode/cli/cmd/spx/main.go

@@ -111,6 +111,20 @@ func buildRootCmd() *cobra.Command {
 	return root
 }
 
+// markerTypeToStateField maps a `spx issue marker --type` value to the state
+// JSON field it mirrors. All three types share one incremental-update path;
+// this table is the single source of truth — adding a type only touches it
+// (plus markerTypesHint below, which is what the user sees).
+var markerTypeToStateField = map[string]string{
+	"spec":      "specFile",
+	"plan":      "planFile",
+	"prototype": "prototypeFile",
+}
+
+// markerTypesHint is the human-readable enumeration used in help text and the
+// "bad --type" error, kept next to the table so the two can't drift.
+const markerTypesHint = "spec、plan 或 prototype"
+
 func buildIssueCmd() *cobra.Command {
 	issue := &cobra.Command{
 		Use:   "issue",
@@ -215,17 +229,17 @@ func buildIssueCmd() *cobra.Command {
 
 	markerCmd := &cobra.Command{
 		Use:   "marker",
-		Short: "增量更新现有工单的 spec/plan marker，并同步合并 specFile/planFile 进 state JSON",
+		Short: "增量更新现有工单的 spec/plan/prototype marker，并同步合并 specFile/planFile/prototypeFile 进 state JSON",
 		RunE: func(cmd *cobra.Command, args []string) error {
 			if imIssue <= 0 {
 				return fmt.Errorf("--issue 必须是正整数")
 			}
-			if imType != "spec" && imType != "plan" {
-				return fmt.Errorf("--type 必须是 spec 或 plan，得到 %q", imType)
+			fieldName, ok := markerTypeToStateField[imType]
+			if !ok {
+				return fmt.Errorf("--type 必须是 %s，得到 %q", markerTypesHint, imType)
 			}
 			// Fail fast on a bad path before we touch the network — the
-			// schema's specFile/planFile pattern doubles as input check.
-			fieldName := map[string]string{"spec": "specFile", "plan": "planFile"}[imType]
+			// schema's pattern for this field doubles as input check.
 			patch := map[string]any{fieldName: imValue}
 			if err := state.ValidateValue(patch); err != nil {
 				return err
@@ -263,8 +277,8 @@ func buildIssueCmd() *cobra.Command {
 		},
 	}
 	markerCmd.Flags().IntVar(&imIssue, "issue", 0, "工单编号（必填）")
-	markerCmd.Flags().StringVar(&imType, "type", "", "marker 类型：spec 或 plan（必填）")
-	markerCmd.Flags().StringVar(&imValue, "value", "", "marker 值（路径，必填，同步写入 state JSON 的 specFile/planFile）")
+	markerCmd.Flags().StringVar(&imType, "type", "", "marker 类型："+markerTypesHint+"（必填）")
+	markerCmd.Flags().StringVar(&imValue, "value", "", "marker 值（路径，必填，同步写入 state JSON 的 specFile/planFile/prototypeFile）")
 	_ = markerCmd.MarkFlagRequired("issue")
 	_ = markerCmd.MarkFlagRequired("type")
 	_ = markerCmd.MarkFlagRequired("value")
@@ -394,11 +408,14 @@ var knownStateFields = map[string]struct{}{
 	"implementSessionId":    {},
 	"reviewSessionId":       {},
 	"testSessionId":         {},
+	"prototypeSessionId":    {},
 	"profilePath":           {},
 	"brainstormProfilePath": {},
 	"testProfilePath":       {},
+	"prototypeProfilePath":  {},
 	"specFile":              {},
 	"planFile":              {},
+	"prototypeFile":         {},
 	"prDiffFile":            {},
 	"pr":                    {},
 	"prMerged":              {},