While Astanga yoga speaks much about reality and nature is much detailed fashion , I hardly see any reference that can qualify theism. Is it an agnostic philosophy or I am just getting it all wrong?
The word Yoga itself means to unite with the absolute or Brahman. Therefore, any form of Yoga cannot be agnostic by nature. However, the definition of Brahman can be very broad and all-encompassing. Because of this, people practicing Ashtanga Yoga can have doubts whether it is atheistic.
Although one of the basics (Yamas-Niyamas) require one to follow Brahmacharya (celibacy; one who follows the Brahman) and Ishvarapranidhana (surrender to God), the path of Asthanga Yoga gears more for the actual "practitioner" as opposed to "believer". The path of Bhakti (devotion) is purely based on higher love for the divine however Asthanga Yoga is based on practical; which means if you do this and this you will naturally head in this direction.
In order to answer your question, one must first define what really is "theism". If your understanding of "theism" is simply blind-belief then obviously NO - Ashtanga Yoga is nothing close to blind-beliefs. It is a path, one must practice the path if one wishes to attain results. But the one who has unshakable faith in the one beyond the 3 worlds, he will definitely progress faster in the path of Spirituality (any path of Yoga).
I see that you have specifically coined the phrase "agnostic philosophy". So the answer would be absolutely no. Because Yoga is not philosophy, it is like science. One gains/attains the fruit of his sadhana (practice), of this there is no doubt.
No, the yoga school of thought that prescribes the astanga yoga is theistic in nature and accepts the existence of God. Sage Patanjali was the prooponent of the yoga school of thought and in his yoga sutras he talks about meditation on God (iśvara praṇidhānāt vā - 1.23 ) and defines God in the following way:
The Supreme Lord is that special person who is not affected by troubles, actions, developments or by subconscious motivations. [PYS - 1.24]
In God, the seed of omniscience is unsurpassed. [PYS - 1.25]
Not being conditioned by time, God is the teacher of even the ancients. [PYS - 1.26]
The yoga school of thought accepts all the principle and philosophy of Sankhya School of Thought which is atheistic in nature, but adds another element to it, Iswara, making it a theistic school.