stage-3c: unsloth QLoRA training scaffold for Qwen3.5

Dockerfile.train builds a CUDA 12.4 + unsloth container that consumes the
Trainline JSONL datasets and emits a LoRA adapter at data/adapters/<run>/final.
Defaults target a 24 GB GPU (Qwen3.5-4B-Instruct-bnb-4bit, r=16, bf16, 3 epochs,
effective batch 8). README documents the build + run workflow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,9 @@
+.venv/
+.git/
+data/
+docs/archive/
+__pycache__/
+*.pyc
+*.egg-info/
+.idea/
+.vscode/