add black-box debug harness

internal/harness/mcp_client.go

@@ -0,0 +1,82 @@
+package harness
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"net"
+	"sync"
+)
+
+type MCPClient struct {
+	conn net.Conn
+	r    *bufio.Reader
+	mu   sync.Mutex
+	next int
+}
+
+type RPCError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+	Data    any    `json:"data,omitempty"`
+}
+
+func (e *RPCError) Error() string {
+	return fmt.Sprintf("json-rpc error %d: %s", e.Code, e.Message)
+}
+
+func DialMCP(socket string) (*MCPClient, error) {
+	conn, err := net.Dial("unix", socket)
+	if err != nil {
+		return nil, err
+	}
+	if _, err := conn.Write([]byte("{\"patterm_identity\":\"harness\"}\n")); err != nil {
+		_ = conn.Close()
+		return nil, err
+	}
+	return &MCPClient{conn: conn, r: bufio.NewReader(conn)}, nil
+}
+
+func (c *MCPClient) Close() error {
+	if c == nil || c.conn == nil {
+		return nil
+	}
+	return c.conn.Close()
+}
+
+func (c *MCPClient) Call(method string, params any) (json.RawMessage, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.next++
+	req := map[string]any{
+		"jsonrpc": "2.0",
+		"id":      c.next,
+		"method":  method,
+	}
+	if params != nil {
+		req["params"] = params
+	}
+	b, err := json.Marshal(req)
+	if err != nil {
+		return nil, err
+	}
+	b = append(b, '\n')
+	if _, err := c.conn.Write(b); err != nil {
+		return nil, err
+	}
+	line, err := c.r.ReadBytes('\n')
+	if err != nil {
+		return nil, err
+	}
+	var resp struct {
+		Result json.RawMessage `json:"result"`
+		Error  *RPCError       `json:"error"`
+	}
+	if err := json.Unmarshal(line, &resp); err != nil {
+		return nil, fmt.Errorf("parse response: %w: %s", err, string(line))
+	}
+	if resp.Error != nil {
+		return nil, resp.Error
+	}
+	return resp.Result, nil
+}