feat: wire Sam21Client into _deps and config

Adds services.sam21 (base_url + timeout_s) and grasp.use_sam21 toggle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/presa_mano/clients/__init__.py

@@ -3,9 +3,12 @@
 from .g1 import G1Client, G1StatusError, G1Unavailable
 from .marker import MarkerClient
 from .nuwa import NuwaClient, NuwaError
+from .sam21 import Sam21Client, Sam21Error, Sam21Unavailable
 from .yolo import YoloClient
 
 __all__ = [
     "G1Client", "G1StatusError", "G1Unavailable",
-    "MarkerClient", "NuwaClient", "NuwaError", "YoloClient",
+    "MarkerClient", "NuwaClient", "NuwaError",
+    "Sam21Client", "Sam21Error", "Sam21Unavailable",
+    "YoloClient",
 ]