refactor: rename stock-lab → stock (graduation)

- git mv stock-lab/ → stock/
- docker-compose.yml: 서비스 키 + container_name + build.context +
  frontend.depends_on + agent-office STOCK_LAB_URL → STOCK_URL
- agent-office/app: config.py, service_proxy.py, agents/stock.py, tests/
  STOCK_LAB_URL → STOCK_URL
- nginx/default.conf: proxy_pass http://stock-lab → http://stock (3 lines)
- CLAUDE.md / README.md / STATUS.md / scripts/ 문구 갱신
- stock/ 내부 자기 참조 갱신

lab 네이밍 정책 (feedback_lab_naming.md) graduation.
API URL / Python import / DB 파일명 변경 없음.

stock/app/screener/nodes/foreign_buy.py

@@ -0,0 +1,33 @@
+"""외국인 N일 누적 순매수 강도 (시총 대비)."""
+
+import pandas as pd
+
+from .base import ScoreNode, percentile_rank
+
+
+class ForeignBuy(ScoreNode):
+    name  = "foreign_buy"
+    label = "외국인 누적 순매수"
+    default_params = {"window_days": 5}
+    param_schema = {
+        "type": "object",
+        "properties": {
+            "window_days": {"type": "integer", "minimum": 1, "maximum": 60, "default": 5}
+        },
+    }
+
+    def compute(self, ctx, params: dict) -> pd.Series:
+        window = int(params.get("window_days", 5))
+        flow = ctx.flow
+        if flow.empty:
+            return pd.Series(dtype=float)
+
+        last_dates = (
+            flow.sort_values("date").groupby("ticker").tail(window)
+        )
+        net_sum = last_dates.groupby("ticker")["foreign_net"].sum()
+
+        market_cap = ctx.master["market_cap"].fillna(0).reindex(net_sum.index)
+        raw = (net_sum / market_cap.replace(0, pd.NA)).astype(float)
+
+        return percentile_rank(raw).fillna(50.0)