Expose the step directory and a tool-call cap

Why: Host tools need the kernel workspace to register files the step just wrote, and product budgets must stop a step that keeps calling tools.
Evidence: go test ./adapters/native/ ./runtime/ -count=1 passed, including the workdir visibility test.
Impact: Host tools can read WorkDir from context. MaxToolCalls limits one kernel step when the request budget leaves it unset.
Refs: deliverable-quality-system B1
Co-authored-by: Cursor <cursoragent@cursor.com>

adapters/native/backend.go

@@ -52,6 +52,8 @@ type Options struct {
 	Subagent subagent.Factory
 	// HostTools 是宿主产品工具（登记工件、卡片、知识库）。与内核工具同名时失败，不覆盖。
 	HostTools []tools.Tool
+	// MaxToolCalls 是单步工具调用上限。0 表示不限制。请求自带预算时以请求为准。
+	MaxToolCalls int
 	// Log 可选。nil 时丢弃日志。
 	Log *logsys.Logger
 	// ExecutionID 写入日志，不参与执行控制。
@@ -77,6 +79,19 @@ type Backend struct {
 	lastFailure map[string]string
 }
 
+type workDirKey struct{}
+
+// WithWorkDir 把本步工作目录放进上下文，供宿主工具读取内核刚写出的文件。
+func WithWorkDir(ctx context.Context, dir string) context.Context {
+	return context.WithValue(ctx, workDirKey{}, dir)
+}
+
+// WorkDir 返回 WithWorkDir 写入的目录。没有时返回空串。
+func WorkDir(ctx context.Context) string {
+	dir, _ := ctx.Value(workDirKey{}).(string)
+	return dir
+}
+
 // New 创建 native 后端。
 func New(opts Options) *Backend {
 	return &Backend{opts: opts, lastFailure: map[string]string{}}
@@ -205,6 +220,9 @@ func (b *Backend) Execute(ctx context.Context, req exec.Request) (*exec.Result,
 		timeout = b.opts.Timeout
 	}
 	var runBudget *runtime.Budget
+	if b.opts.MaxToolCalls > 0 {
+		runBudget = &runtime.Budget{MaxToolCalls: b.opts.MaxToolCalls}
+	}
 	if req.Budget != nil {
 		runBudget = &runtime.Budget{
 			MaxPromptTokens:     req.Budget.MaxPromptTokens,
@@ -212,6 +230,9 @@ func (b *Backend) Execute(ctx context.Context, req exec.Request) (*exec.Result,
 			MaxToolCalls:        req.Budget.MaxToolCalls,
 			Deadline:            req.Budget.Deadline,
 		}
+		if runBudget.MaxToolCalls == 0 {
+			runBudget.MaxToolCalls = b.opts.MaxToolCalls
+		}
 		if !req.Budget.Deadline.IsZero() {
 			// Deadline 与 Timeout 先到者生效：把更短者作为 ctx 超时。
 			if until := time.Until(req.Budget.Deadline); timeout <= 0 || until < timeout {
@@ -219,10 +240,10 @@ func (b *Backend) Execute(ctx context.Context, req exec.Request) (*exec.Result,
 			}
 		}
 	}
-	runCtx := ctx
+	runCtx := WithWorkDir(ctx, workDir)
 	cancel := func() {}
 	if timeout > 0 {
-		runCtx, cancel = context.WithTimeout(ctx, timeout)
+		runCtx, cancel = context.WithTimeout(runCtx, timeout)
 	}
 	defer cancel()