cleanup: drop dead ApiClient::stream_completion wrapper, silence dmn_tick

stream_completion was a thin wrapper around stream_completion_mm (just
passing an empty image list); the last caller switched to _mm directly
when learn's generate_alternate gained image support. Delete the
wrapper — callers can pass `&[]` if they have no images.

MindState::dmn_tick has been sitting unused (called only from a
commented-out block in the Mind loop). Rename to _dmn_tick so the
compiler stops warning; Kent may uncomment the call path later.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -73,15 +73,6 @@ impl ApiClient {
         }
     }
 
-    pub(crate) fn stream_completion(
-        &self,
-        prompt_tokens: &[u32],
-        sampling: SamplingParams,
-        priority: Option<i32>,
-    ) -> (mpsc::UnboundedReceiver<StreamToken>, AbortOnDrop) {
-        self.stream_completion_mm(prompt_tokens, &[], sampling, priority)
-    }
-
     pub(crate) fn stream_completion_mm(
         &self,
         prompt_tokens: &[u32],