Let hosts register tools and observe token usage

Why: product runners need register_artifact and card tools inside the kernel loop, and the UI needs token usage on the same event stream as text and tool calls.
Evidence: go test ./... in agent-native; TestHostToolAndUsageEvent and TestHostToolNameConflictFailsClosed.
Impact: Options.HostTools fail closed on name clashes; ProcessEvent and runtime events gain a usage type with token counts. Existing callers that ignore unknown event types stay compatible.
Refs: deliverable-quality-system B1
Co-authored-by: Cursor <cursoragent@cursor.com>

exec/backend.go

@@ -40,9 +40,9 @@ type Request struct {
 // 共享总预算与截止时间，避免传输、步骤与主循环次数相乘）。
 // 全部字段可选；零值字段不设限。
 type Budget struct {
-	MaxPromptTokens     int       `json:"max_prompt_tokens,omitempty"`
-	MaxCompletionTokens int       `json:"max_completion_tokens,omitempty"`
-	MaxToolCalls        int       `json:"max_tool_calls,omitempty"`
+	MaxPromptTokens     int `json:"max_prompt_tokens,omitempty"`
+	MaxCompletionTokens int `json:"max_completion_tokens,omitempty"`
+	MaxToolCalls        int `json:"max_tool_calls,omitempty"`
 	// Deadline 绝对截止时刻；与 Request.Timeout 先到者生效。
 	Deadline time.Time `json:"deadline,omitempty"`
 }
@@ -54,6 +54,9 @@ type ProcessEvent struct {
 	Text   string // delta 文本（Type 为 *_delta 时）
 	Tool   string // 工具名（tool_* 时）
 	Detail string // 工具参数/结果摘要（tool_execution_* 时）
+	// PromptTokens / CompletionTokens 在 Type=usage 时填写。
+	PromptTokens     int
+	CompletionTokens int
 }
 
 const (