Rewrite Settings as native Periscope config.

DeepSeek remains the only live provider; leftover Anthropic env names
are coerced or ignored. Skills manifest is resolved across the split tree.

tests/test_periscope_config_rewrite.py

@@ -0,0 +1,50 @@
+"""Native Settings lives under periscope/src and keeps DeepSeek routing."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import backend.config as config
+from backend.config import Settings, settings
+
+
+def test_config_module_is_src():
+    path = Path(config.__file__).resolve()
+    assert path.name == "config.py"
+    assert "src" in path.parts
+    assert "Native Periscope overlay" not in path.read_text(encoding="utf-8")[:400]
+
+
+def test_deepseek_defaults_and_anthropic_coercion():
+    assert Settings.model_fields["provider_default"].default == "deepseek"
+    assert Settings.model_fields["deepseek_model"].default == "deepseek-flash"
+    assert Settings.model_fields["model_pintable_deepseek"].default == "deepseek-flash"
+    assert settings.provider_for_stage("pintable") == "deepseek"
+
+    snap = settings.provider_validation
+    try:
+        settings.provider_validation = "anthropic"
+        assert settings.provider_for_stage("validation") == "deepseek"
+        assert "deepseek" in settings.model_for_stage("validation")
+    finally:
+        settings.provider_validation = snap
+
+
+def test_anthropic_fallback_is_none():
+    snap_p = settings.fallback_provider_validation
+    snap_m = settings.fallback_model_validation
+    try:
+        settings.fallback_provider_validation = "anthropic"
+        settings.fallback_model_validation = "claude-sonnet-4-6"
+        assert settings.fallback_for_stage("validation") is None
+    finally:
+        settings.fallback_provider_validation = snap_p
+        settings.fallback_model_validation = snap_m
+
+
+def test_skills_manifest_is_readable():
+    version = settings.get_default_model_version()
+    assert version
+    skill_id, latest = settings.get_skill("extract-pintable")
+    assert skill_id.startswith("skill_")
+    assert latest