fix(scripts): align build_trees.sh with project shell conventions

Codex 审查发现两处偏离既有脚本惯例：
1. conda activate 改为 PYTHON=$(conda run ...) + ${PYTHON} 模式
2. 追加 source .env 确保 TREE_BUILD_API_CONCURRENCY 等工程配置可用

scripts/build_trees.sh

@@ -28,11 +28,12 @@ VIDEO_CONCURRENCY="${VIDEO_CONCURRENCY:-16}"
 
 export PYTHONUNBUFFERED=1
 
-# shellcheck source=/dev/null
-source "$(conda info --base)/etc/profile.d/conda.sh"
-conda activate Video-Tree-TRM
+# shellcheck source=../.env
+source .env
 
-python tools/build_trees.py \
+PYTHON="$(conda run -n Video-Tree-TRM which python)"
+
+"${PYTHON}" tools/build_trees.py \
     --out-dir store/videos \
     --video-concurrency "$VIDEO_CONCURRENCY" \
     "$@"