Initial commit: hardened DeerFlow factory

Vendored deer-flow upstream (bytedance/deer-flow) plus prompt-injection
hardening:

- New deerflow.security package: content_delimiter, html_cleaner,
  sanitizer (8 layers — invisible chars, control chars, symbols, NFC,
  PUA, tag chars, horizontal whitespace collapse with newline/tab
  preservation, length cap)
- New deerflow.community.searx package: web_search, web_fetch,
  image_search backed by a private SearX instance, every external
  string sanitized and wrapped in <<<EXTERNAL_UNTRUSTED_CONTENT>>>
  delimiters
- All native community web providers (ddg_search, tavily, exa,
  firecrawl, jina_ai, infoquest, image_search) replaced with hard-fail
  stubs that raise NativeWebToolDisabledError at import time, so a
  misconfigured tool.use path fails loud rather than silently falling
  back to unsanitized output
- Native client back-doors (jina_client.py, infoquest_client.py)
  stubbed too
- Native-tool tests quarantined under tests/_disabled_native/
  (collect_ignore_glob via local conftest.py)
- Sanitizer Layer 7 fix: only collapse horizontal whitespace, preserve
  newlines and tabs so list/table structure survives
- Hardened runtime config.yaml references only the searx-backed tools
- Factory overlay (backend/) kept in sync with deer-flow tree as a
  reference / source

See HARDENING.md for the full audit trail and verification steps.

deer-flow/backend/tests/test_security_scanner.py

@@ -0,0 +1,17 @@
+from types import SimpleNamespace
+
+import pytest
+
+from deerflow.skills.security_scanner import scan_skill_content
+
+
+@pytest.mark.anyio
+async def test_scan_skill_content_blocks_when_model_unavailable(monkeypatch):
+    config = SimpleNamespace(skill_evolution=SimpleNamespace(moderation_model_name=None))
+    monkeypatch.setattr("deerflow.skills.security_scanner.get_app_config", lambda: config)
+    monkeypatch.setattr("deerflow.skills.security_scanner.create_chat_model", lambda **kwargs: (_ for _ in ()).throw(RuntimeError("boom")))
+
+    result = await scan_skill_content("---\nname: demo-skill\ndescription: demo\n---\n", executable=False)
+
+    assert result.decision == "block"
+    assert "manual review required" in result.reason