Add DeepSeek harness in t3code.

[MDHaarith]

I have use many harnesses for my works. But none of them is as cheap as DeepSeek harness. It handles cache hit more better than any other harness. I use my opencode-go subscription via DeepSeek harness and some session I get 99.2% chache hit rate and the lowest ever is 97.3% . But the same in opencode is around 94% to 96%. This is a huge saving when I use 100M tokens.

[juliusmarminge]

Closing as a duplicate of #11937 — same request to add DeepSeek harness support.

Please continue the conversation there (related earlier pulse: #7331).