✨ 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/state/schema_test.go

@@ -71,11 +71,14 @@ func TestValidateAcceptsFullState(t *testing.T) {
         "implementSessionId":"impl-1",
         "reviewSessionId":"rev-1",
         "testSessionId":"test-1",
+        "prototypeSessionId":"proto-1",
         "profilePath":"/home/x/.claude/settings.json",
         "brainstormProfilePath":"/home/x/.claude/profiles/brainstorm.json",
         "testProfilePath":"/home/x/.claude/profiles/test.json",
+        "prototypeProfilePath":"/home/x/.claude/profiles/prototype.json",
         "specFile":"docs/superpowers/specs/foo.md",
         "planFile":"docs/superpowers/plans/foo.md",
+        "prototypeFile":"docs/prototype/login/figma-plugin/manifest.json",
         "prDiffFile":"docs/pr-diff/pr-42-issue-7.md",
         "pr":"42",
         "prMerged":true,
@@ -91,6 +94,13 @@ func TestValidateAcceptsFullState(t *testing.T) {
 	}
 }
 
+func TestValidateRejectsBadPrototypeFile(t *testing.T) {
+	err := Validate([]byte(`{"prototypeFile":"docs/prototype/login/figma-plugin/manifest.md"}`))
+	if err == nil {
+		t.Fatal("非 .json 路径应当被 prototypeFile pattern 拒绝")
+	}
+}
+
 func TestValidateAcceptsBrainstormProfilePathOnly(t *testing.T) {
 	if err := Validate([]byte(`{"brainstormProfilePath":"/home/x/.claude/profiles/brainstorm.json"}`)); err != nil {
 		t.Fatalf("仅含 brainstormProfilePath 应当通过校验，得到: %v", err)