fix(worker): increase SSE scanner buffer to 1MB in claudebox client

The HTTP claudebox client's ExecuteStream method used a bare
bufio.NewScanner with the default 64KB max token size. When Claude Code
produces tool results > 64KB (e.g., reading large files), the SSE event
exceeds the scanner limit and fails with "token too long".

Every other scanner in the codebase (claudecode adapter, claudebox
executor, kubernetes executor) already uses scanner.Buffer(buf, 1MB).
This was the only one missed.

Fixes: "agent execution failed: read stream: bufio.Scanner: token too long"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/adapter/claudebox/client.go

@@ -175,6 +175,8 @@ func (c *Client) ExecuteStream(ctx context.Context, req *ExecuteRequest, handler
 
 	// Parse SSE events
 	scanner := bufio.NewScanner(resp.Body)
+	buf := make([]byte, 0, 64*1024)
+	scanner.Buffer(buf, 1024*1024) // 1MB max to match other streaming paths
 	for scanner.Scan() {
 		line := scanner.Text()
 		if !strings.HasPrefix(line, "data: ") {