fix: strip empty env vars in _getenv_num/_getenv_bool; unify FORCE_CPU

- _getenv_num: add raw.strip() + empty-string guard so numeric vars set
  to "" (common Compose pattern for "use default") return the default
  silently instead of warning "not a valid int/float"
- _getenv_bool: same guard so True-defaulted flags set to "" return
  the configured default instead of silently returning False
- embeddings.py: replace inline FORCE_CPU bool parse with _getenv_bool

winnow/config.py

@@ -16,6 +16,9 @@ def _getenv_num(name: str, default, cast):
     raw = os.getenv(name)
     if raw is None:
         return default
+    raw = raw.strip()
+    if not raw:
+        return default
     try:
         return cast(raw)
     except ValueError:
@@ -46,6 +49,9 @@ def _getenv_bool(name: str, default: bool) -> bool:
     raw = os.getenv(name)
     if raw is None:
         return default
+    raw = raw.strip()
+    if not raw:
+        return default
     return raw.lower() in ("true", "1", "yes")