fix(agent): 审计写入失败不再静默

审计日志是证据而不是控制手段：写失败不能让请求失败，否则磁盘写满会变成一次远程
拒绝服务——已经执行成功的指令会因为日志追加不上而开始报错，那是坏错了地方。

但它也不能消失。静默丢审计条目是那种"运维以为自己有记录、其实没有"的失败，而这份
日志的全部意义就是事后能回答"谁在什么时候要求了什么"。现在写失败会进 agent 自己的
日志，磁盘写满表现为一行反复出现，而不是一片空白。

顺带把 Close 之后的语义写清楚：Close 会丢掉 sink，此后的写入返回 nil——这是刻意的，
因为一个正在关闭的进程与在途请求赛跑时，为每一个恰好到达的请求报一次失败是噪声
而不是信息。这条差别在 agent 日志里看得见。

测试用真实失败路径（写一个已关闭的文件）而不是填满文件系统，并断言 exec.record 把
它报了出来。

Go 全包、Node 112 项、Swift 101 项通过。

internal/runtime/runtime_test.go

@@ -1,9 +1,11 @@
 package runtime
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -544,3 +546,51 @@ func TestExecutorRefusesToStartWithoutAPolicyOrPort(t *testing.T) {
 		t.Fatal("port 0 was accepted")
 	}
 }
+
+// A failed audit write must be visible somewhere. The entry cannot fail the request — the audit log is
+// evidence, not a control, and a full disk must not become a remote denial of service — but silently
+// losing entries is the failure mode where an operator believes they have a record and does not.
+func TestAFailedAuditWriteIsReported(t *testing.T) {
+	directory := t.TempDir()
+	path := filepath.Join(directory, "audit.log")
+	audit, err := OpenAudit(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Replace the sink with one that fails, which is what a full disk looks like from here. Reaching
+	// into the struct is deliberate: the alternative is filling a filesystem in a test.
+	audit.mu.Lock()
+	audit.sink = nil
+	audit.mu.Unlock()
+
+	var captured bytes.Buffer
+	previous := slog.Default()
+	slog.SetDefault(slog.New(slog.NewJSONHandler(&captured, &slog.HandlerOptions{Level: slog.LevelDebug})))
+	defer slog.SetDefault(previous)
+
+	// Close() drops the sink, so a write after it returns nil rather than an error — that is the
+	// documented shutdown case. Force the error path by giving it a closed file instead.
+	file, err := os.OpenFile(filepath.Join(directory, "closed.log"), os.O_CREATE|os.O_WRONLY, 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := file.Close(); err != nil {
+		t.Fatal(err)
+	}
+	audit.mu.Lock()
+	audit.sink = file
+	audit.mu.Unlock()
+
+	if err := audit.Write(Entry{Node: "nd_x", Req: "r1", Outcome: "ok"}); err == nil {
+		t.Fatal("writing to a closed file reported success")
+	}
+	// And the executor's own path reports it rather than swallowing it.
+	exec, err := NewExecutor(testPolicy(t), Config{Port: 3000, Audit: audit})
+	if err != nil {
+		t.Fatal(err)
+	}
+	exec.record(wire.Envelope{Node: "nd_x", Req: "r2", App: "ap_x"}, "GET", "/api/health", 200, "ok")
+	if !strings.Contains(captured.String(), "audit entry could not be written") {
+		t.Fatalf("a failed audit write was not reported; log was: %s", captured.String())
+	}
+}