fix(renovate): include sha256: prefix in openclaw digest autoReplace template

The autoReplaceStringTemplate was outputting {{{newDigest}}} which
resolves to just the hex hash (e.g. 04e2738...) without the sha256:
prefix. Since the regex expects to match "sha256:(?<currentDigest>)"
and the file stores digests as "sha256:HASH", the replacement produced
a digest value missing the prefix. Renovate detected this mismatch
("Digest is not updated") and failed with "Error updating branch:
update failure".

renovate.json

@@ -29,7 +29,7 @@
       "matchStrings": [
         "  # renovate: datasource=(?<datasource>[^\\s]+) depName=(?<depName>[^\\s]+)\\n  openclawImageTag = \"(?<currentValue>[^\"]+)\";\\n  openclawImageDigest = \"sha256:(?<currentDigest>[^\"]+)\";"
       ],
-      "autoReplaceStringTemplate": "  # renovate: datasource={{{datasource}}} depName={{{depName}}}\n  openclawImageTag = \"{{{newValue}}}\";\n  openclawImageDigest = \"{{{newDigest}}}\";"
+      "autoReplaceStringTemplate": "  # renovate: datasource={{{datasource}}} depName={{{depName}}}\n  openclawImageTag = \"{{{newValue}}}\";\n  openclawImageDigest = \"sha256:{{{newDigest}}}\";"
     }
   ],
   "packageRules": [