feat: auto-avatar con proveedores gratuitos (DiceBear, RoboHash, Multiavatar)

Nuevo sistema de generacion automatica de avatares:

- pkg/avatar/ — tipos puros y URL builders para proveedores gratuitos:
  DiceBear (bottts, pixel-art, etc.), RoboHash (robots, monsters),
  Multiavatar (multicultural). Sin I/O.
- shell/avatar/ — fetcher impuro: descarga imagen por HTTP a temp file.
- agentctl auto-avatar <id> — genera, descarga, sube y activa avatar
  con un solo comando. Soporta --provider, --style, --set, --dry-run.

Respeta pure core / impure shell. El seed del avatar es el agent ID,
asi cada bot obtiene una imagen unica y determinista.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shell/avatar/fetch.go

@@ -0,0 +1,130 @@
+// Package avatar fetches avatar images from external providers.
+// Impure shell: performs HTTP I/O.
+package avatar
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/enmanuel/agents/pkg/avatar"
+)
+
+const (
+	fetchTimeout = 30 * time.Second
+	maxBytes     = 10 * 1024 * 1024 // 10 MB
+)
+
+// Fetch downloads an avatar image from the given provider using seed as the
+// generation key (typically the agent ID or display name). Returns the path
+// to a temporary file containing the image.
+// The caller is responsible for removing the temp file when done.
+func Fetch(ctx context.Context, provider avatar.Provider, seed string, opts avatar.Options) (string, error) {
+	imageURL := avatar.URL(provider, seed, opts)
+	return Download(ctx, imageURL)
+}
+
+// Download fetches the image at imageURL and writes it to a temp file.
+// Returns the temp file path. The caller must remove the file when done.
+func Download(ctx context.Context, imageURL string) (string, error) {
+	ctx, cancel := context.WithTimeout(ctx, fetchTimeout)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, imageURL, nil)
+	if err != nil {
+		return "", fmt.Errorf("create request: %w", err)
+	}
+	req.Header.Set("User-Agent", "agents-avatar-fetcher/1.0")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("fetch %s: %w", imageURL, err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("fetch %s: HTTP %d", imageURL, resp.StatusCode)
+	}
+
+	ext := extensionFromContentType(resp.Header.Get("Content-Type"))
+
+	tmp, err := os.CreateTemp("", "avatar-*"+ext)
+	if err != nil {
+		return "", fmt.Errorf("create temp file: %w", err)
+	}
+
+	if _, err := io.Copy(tmp, io.LimitReader(resp.Body, maxBytes)); err != nil {
+		tmp.Close()
+		os.Remove(tmp.Name())
+		return "", fmt.Errorf("write temp file: %w", err)
+	}
+
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmp.Name())
+		return "", fmt.Errorf("close temp file: %w", err)
+	}
+
+	return tmp.Name(), nil
+}
+
+// FetchToDir downloads the avatar and saves it to dir/<seed>.<ext>.
+// Creates dir if it doesn't exist. Returns the final file path.
+func FetchToDir(ctx context.Context, provider avatar.Provider, seed string, opts avatar.Options, dir string) (string, error) {
+	tmpPath, err := Fetch(ctx, provider, seed, opts)
+	if err != nil {
+		return "", err
+	}
+	defer os.Remove(tmpPath)
+
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", fmt.Errorf("create dir %s: %w", dir, err)
+	}
+
+	ext := filepath.Ext(tmpPath)
+	finalPath := filepath.Join(dir, seed+ext)
+
+	src, err := os.Open(tmpPath)
+	if err != nil {
+		return "", fmt.Errorf("open temp: %w", err)
+	}
+	defer src.Close()
+
+	dst, err := os.Create(finalPath)
+	if err != nil {
+		return "", fmt.Errorf("create %s: %w", finalPath, err)
+	}
+
+	if _, err := io.Copy(dst, src); err != nil {
+		dst.Close()
+		os.Remove(finalPath)
+		return "", fmt.Errorf("copy to %s: %w", finalPath, err)
+	}
+
+	if err := dst.Close(); err != nil {
+		os.Remove(finalPath)
+		return "", err
+	}
+
+	return finalPath, nil
+}
+
+func extensionFromContentType(ct string) string {
+	switch ct {
+	case "image/png":
+		return ".png"
+	case "image/jpeg":
+		return ".jpg"
+	case "image/svg+xml":
+		return ".svg"
+	case "image/webp":
+		return ".webp"
+	case "image/gif":
+		return ".gif"
+	default:
+		return ".png"
+	}
+}