fix(ci): derive tag from git when CM_TAG is unset

Codemagic doesn't set CM_TAG for Gitea webhook integrations.
Fall back to git tag --points-at HEAD.

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

codemagic.yaml

@@ -62,7 +62,8 @@ workflows:
           echo "TAG=$CM_TAG  BUILD_DIR=$CM_BUILD_DIR"
           echo "GITEA_API=$GITEA_API  TOKEN_SET=yes"
 
-          TAG="$CM_TAG"
+          TAG="${CM_TAG:-$(git tag --points-at HEAD | head -1)}"
+          [ -n "$TAG" ] || { echo "ERROR: could not determine tag"; exit 1; }
           VERSION="${TAG#v}"
           IPA="$CM_BUILD_DIR/SilverApple.ipa"