fix(cookbook): locate cookbook_state.json via DATA_DIR, not hardcoded /app/data (#3332)

Three call sites hardcoded Path("/app/data/cookbook_state.json"), which only
exists in Docker; on a native run the real path is <repo>/data, so the state
file looked missing and cookbook serve-state was silently ignored. Two others
used os.environ.get("DATA_DIR", "data") (a relative fallback, since DATA_DIR is
never set as an env var). Route all five through core.constants.DATA_DIR so the
path is consistent and absolute on both Docker and native.

Part of #3331.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_cookbook_state_path.py

@@ -0,0 +1,29 @@
+"""Guard: cookbook_state.json must be located via DATA_DIR, not hardcoded /app/data
+(which breaks native runs) or a relative os.environ fallback."""
+import pathlib
+
+ROOT = pathlib.Path(__file__).resolve().parent.parent
+FILES = [
+    "src/cookbook_serve_lifecycle.py",
+    "src/builtin_actions.py",
+    "routes/codex_routes.py",
+    "routes/cookbook_routes.py",
+]
+
+
+def test_no_hardcoded_app_data_cookbook_state():
+    for rel in FILES:
+        text = (ROOT / rel).read_text(encoding="utf-8")
+        for ln in text.splitlines():
+            if ln.strip().startswith("#"):
+                continue
+            assert "/app/data/cookbook_state" not in ln, f"{rel}: hardcoded /app/data: {ln.strip()}"
+            assert 'os.environ.get("DATA_DIR"' not in ln, f"{rel}: relative DATA_DIR env fallback: {ln.strip()}"
+
+
+def test_cookbook_state_uses_datadir_constant():
+    # Each file that references cookbook_state.json should import the DATA_DIR constant.
+    for rel in FILES:
+        text = (ROOT / rel).read_text(encoding="utf-8")
+        if "cookbook_state.json" in text:
+            assert "from core.constants import DATA_DIR" in text, f"{rel}: missing DATA_DIR import"