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/executor.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
 	"strings"
 	"sync"
@@ -261,11 +262,21 @@ func (e *Executor) refuse(envelope wire.Envelope, inner wire.InnerRequest, class
 	return &Refusal{Class: class, Path: inner.Path, Err: err}
 }
 
+// record appends one audit entry, and reports a failure to the process log.
+//
+// The write error is not propagated to the caller: the audit log is evidence, not a control, and a
+// full disk must not become a remote denial of service — a request that already succeeded would start
+// failing because the log could not be appended to, which is the wrong thing to break.
+//
+// But it must not vanish either. Silently losing audit entries is the failure mode where an operator
+// believes they have a record and does not, and the whole point of the log is to be able to answer
+// "who asked for what" afterwards. So the error goes to the agent's own log, where a full disk shows
+// up as a repeating line rather than as an absence.
 func (e *Executor) record(envelope wire.Envelope, method, path string, status int, outcome string) {
 	if e.audit == nil {
 		return
 	}
-	e.audit.Write(Entry{
+	if err := e.audit.Write(Entry{
 		At:      e.clock(),
 		Node:    envelope.Node,
 		Req:     envelope.Req,
@@ -274,7 +285,9 @@ func (e *Executor) record(envelope wire.Envelope, method, path string, status in
 		Path:    path,
 		Status:  status,
 		Outcome: outcome,
-	})
+	}); err != nil {
+		slog.Warn("audit entry could not be written", "error", err, "req", envelope.Req, "outcome", outcome)
+	}
 }
 
 // rateLimiter is a sliding-window counter over the dangerous writes this node has performed.