chore: untrack pretrained/checkpoint weights and server logs, fix yolo26n run path

- Track only trained best.pt under runs/*/weights; ignore epoch/last
  checkpoints, downloaded base weights, and server logs (.gitignore)
- Move runs/detect/runs/detect/smoke_fire_yolo26n_v2-3 to
  runs/detect/smoke_fire_yolo26n_v2-3 and update docs/model_comparison.md
- Replace the Git LFS claim with an accurate plain-file statement

.gitignore

@@ -15,10 +15,16 @@ wheels/
 .idea/
 .vscode/
 
-# Local datasets and training outputs
+# Local datasets and generated training outputs
 data/*
 
-# Model artifacts
+# Model artifacts: ignore checkpoints and downloaded base weights;
+# trained best.pt under any runs/*/weights/ stays trackable
+*.pt
+!**/weights/best.pt
 *.onnx
 *.engine
 *.torchscript
+
+# Logs
+*.log