fix(plugin): /api/ui-inject 的 stages 漏掉 chain_step + 忽略临时构建目录

部署到线上时用隔离实例（独立端口 18099 + 独立 config/runtime/adapter 目录）
发真实请求验证，抓到的第三个 bug。

## bug：discovery 文档漏掉新 stage
handlePluginUI 的响应里 stages 是**字面写死的三个**。加 chain_step 时只改了
lua.AllStages，没改这里，于是插件作者读 GET /api/ui-inject 会看到
["request_start","routed","request_end"]，**合理地得出结论：没有 chain_step 这个
stage**。stage 本身是注册好的、也确实在触发，只是没被声明。

改成从 lua.AllStages 派生——AllStages 是唯一定义顺序的地方，让它保持唯一。

★ 而我原来的测试断言 `len(view.Stages) != 3`，**断言本身是 bug 的保护伞**：
它把"三个"固化成了期望值，于是新增第四个 stage 时测试全绿、bug 上线。
现在断言改为「与 AllStages 等长且逐项相同」，并显式要求 chain_step 在其中。
新增 stage 而忘了声明，这类问题会立刻红。

## 顺带：.gitignore 补上临时构建目录
.probe/ 和 .build-work/ 是我调试时当 GOTMPDIR 和临时二进制用的，之前每轮
手工删，这轮差点提交进去 9.5MB 的二进制。

## 部署验证留档
隔离实例跑真实 chat（158 prompt / 13 completion / 128 cache_hit），计费插件
算出 0.000688，与手算 (158-128)*1e-5 + 128*1e-5*0.1 + 13*2e-5 **逐位吻合**。
★ 第一次手算我按全价算成 0.00184，一度以为插件算错了——查审计记录才看到
cache_hit_tokens。**算钱不对时先查输入再怀疑实现**，而我忘的恰是刚修的折扣。

## 验证
351 个测试全绿；变异（stages 改回硬编码三个）被 TestUIInjectServesPluginUI
抓住，3 条断言同时红。

.gitignore

@@ -26,3 +26,7 @@ cmd/gui/dist/
 
 # local design/working notes (not part of the shipped repo)
 /plan.md
+
+# scratch build/probe dirs used while debugging (GOTMPDIR, throwaway binaries)
+.build-work/
+.probe/

internal/gateway/plugins_api.go

@@ -51,16 +51,27 @@ func (g *Gateway) handlePluginUI(w http.ResponseWriter, r *http.Request) {
 		writeJSON(w, http.StatusOK, map[string]interface{}{"ui": lua.UIExtension{}})
 		return
 	}
+	// AllStages, not a hand-written list: an earlier version enumerated the
+	// three stages literally here, so when chain_step was added it was silently
+	// missing from this response — a plugin author reading the discovery
+	// document would have believed the stage did not exist. Deriving it from
+	// the one place that defines the order is the whole point of having it.
 	writeJSON(w, http.StatusOK, map[string]interface{}{
-		"ui": ps.UI(),
-		"stages": []string{
-			string(lua.StageRequestStart),
-			string(lua.StageRouted),
-			string(lua.StageRequestEnd),
-		},
+		"ui":     ps.UI(),
+		"stages": pipelineStageNames(),
 	})
 }
 
+// stageNames returns the pipeline stage names in firing order, for the
+// discovery payload and for tests.
+func pipelineStageNames() []string {
+	out := make([]string, 0, len(lua.AllStages))
+	for _, s := range lua.AllStages {
+		out = append(out, string(s))
+	}
+	return out
+}
+
 func (g *Gateway) handlePluginsAPI(w http.ResponseWriter, r *http.Request) {
 	ps := g.core.Plugins()
 	if ps == nil {

internal/gateway/plugins_api_test.go

@@ -97,8 +97,19 @@ func TestUIInjectServesPluginUI(t *testing.T) {
 			t.Errorf("element target = %q, want \"status\"", e.Target)
 		}
 	}
-	if len(view.Stages) != 3 {
-		t.Errorf("stages = %v, want the three pipeline stages", view.Stages)
+	// Derived from AllStages, not hardcoded: the previous assertion of "3"
+	// is exactly what let chain_step go missing from this payload unnoticed.
+	if len(view.Stages) != len(lua.AllStages) {
+		t.Errorf("stages = %v, want %d (one per AllStages entry)", view.Stages, len(lua.AllStages))
+	}
+	for i, st := range lua.AllStages {
+		if i >= len(view.Stages) || view.Stages[i] != string(st) {
+			t.Errorf("stages[%d] = %v, want %q", i, view.Stages, string(st))
+		}
+	}
+	if !containsStr(view.Stages, string(lua.StageChainStep)) {
+		t.Error("the discovery payload does not advertise chain_step; a plugin " +
+			"author would conclude the stage does not exist")
 	}
 }
 
@@ -116,6 +127,16 @@ func TestUIInjectIsEmptyWithoutPlugins(t *testing.T) {
 	}
 }
 
+// containsStr reports whether list has s.
+func containsStr(list []string, s string) bool {
+	for _, x := range list {
+		if x == s {
+			return true
+		}
+	}
+	return false
+}
+
 // newRecorderFor pushes a request through the full handler chain.
 func newRecorderFor(t *testing.T, g *Gateway, req *http.Request) *httptest.ResponseRecorder {
 	t.Helper()