✨ 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/internal/marker/marker_test.go

@@ -29,16 +29,38 @@ func TestUpsertMarkerReplacesWhenPresent(t *testing.T) {
 }
 
 func TestUpsertMarkerLeavesOtherTypesAlone(t *testing.T) {
-	body := "<!-- spx:plan=p.md -->\n<!-- spx:spec=old.md -->"
+	body := "<!-- spx:plan=p.md -->\n<!-- spx:prototype=docs/prototype/x/figma-plugin/manifest.json -->\n<!-- spx:spec=old.md -->"
 	got := UpsertMarker(body, "spec", "new.md")
 	if !strings.Contains(got, "<!-- spx:plan=p.md -->") {
 		t.Fatalf("plan marker should be preserved: %q", got)
 	}
+	if !strings.Contains(got, "<!-- spx:prototype=docs/prototype/x/figma-plugin/manifest.json -->") {
+		t.Fatalf("prototype marker should be preserved: %q", got)
+	}
 	if !strings.Contains(got, "<!-- spx:spec=new.md -->") {
 		t.Fatalf("spec marker should be updated: %q", got)
 	}
 }
 
+func TestUpsertMarkerPrototypeAppendsWhenAbsent(t *testing.T) {
+	got := UpsertMarker("hello body", "prototype", "docs/prototype/login/figma-plugin/manifest.json")
+	want := "hello body\n<!-- spx:prototype=docs/prototype/login/figma-plugin/manifest.json -->"
+	if got != want {
+		t.Fatalf("got %q want %q", got, want)
+	}
+}
+
+func TestUpsertMarkerPrototypeReplacesWhenPresent(t *testing.T) {
+	body := "intro\n<!-- spx:prototype=old/manifest.json -->\nmore"
+	got := UpsertMarker(body, "prototype", "new/manifest.json")
+	if strings.Contains(got, "old/manifest.json") {
+		t.Fatalf("old value should be gone: %q", got)
+	}
+	if !strings.Contains(got, "<!-- spx:prototype=new/manifest.json -->") {
+		t.Fatalf("new marker missing: %q", got)
+	}
+}
+
 func TestUpsertMarkerEmptyBody(t *testing.T) {
 	got := UpsertMarker("", "plan", "p.md")
 	want := "<!-- spx:plan=p.md -->"