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/docs/plans/2026-04-01-langfuse-tracing.md

@@ -0,0 +1,105 @@
+# Langfuse Tracing Implementation Plan
+
+**Goal:** Add optional Langfuse observability support to DeerFlow while preserving existing LangSmith tracing and allowing both providers to be enabled at the same time.
+
+**Architecture:** Extend tracing configuration from a single LangSmith-only shape to a multi-provider config, add a tracing callback factory that builds zero, one, or two callbacks based on environment variables, and update model creation to attach those callbacks. If a provider is explicitly enabled but misconfigured or fails to initialize, tracing initialization during model creation should fail with a clear error naming that provider.
+
+**Tech Stack:** Python 3.12, Pydantic, LangChain callbacks, LangSmith, Langfuse, pytest
+
+---
+
+### Task 1: Add failing tracing config tests
+
+**Files:**
+- Modify: `backend/tests/test_tracing_config.py`
+
+**Step 1: Write the failing tests**
+
+Add tests covering:
+- Langfuse-only config parsing
+- dual-provider parsing
+- explicit enable with missing required Langfuse fields
+- provider enable detection without relying on LangSmith-only helpers
+
+**Step 2: Run tests to verify they fail**
+
+Run: `cd backend && uv run pytest tests/test_tracing_config.py -q`
+Expected: FAIL because tracing config only supports LangSmith today.
+
+**Step 3: Write minimal implementation**
+
+Update tracing config code to represent multiple providers and expose helper functions needed by the tests.
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd backend && uv run pytest tests/test_tracing_config.py -q`
+Expected: PASS
+
+### Task 2: Add failing callback factory and model attachment tests
+
+**Files:**
+- Modify: `backend/tests/test_model_factory.py`
+- Create: `backend/tests/test_tracing_factory.py`
+
+**Step 1: Write the failing tests**
+
+Add tests covering:
+- LangSmith callback creation
+- Langfuse callback creation
+- dual callback creation
+- startup failure when an explicitly enabled provider cannot initialize
+- model factory appends all tracing callbacks to model callbacks
+
+**Step 2: Run tests to verify they fail**
+
+Run: `cd backend && uv run pytest tests/test_model_factory.py tests/test_tracing_factory.py -q`
+Expected: FAIL because there is no provider factory and model creation only attaches LangSmith.
+
+**Step 3: Write minimal implementation**
+
+Create tracing callback factory module and update model factory to use it.
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd backend && uv run pytest tests/test_model_factory.py tests/test_tracing_factory.py -q`
+Expected: PASS
+
+### Task 3: Wire dependency and docs
+
+**Files:**
+- Modify: `backend/packages/harness/pyproject.toml`
+- Modify: `README.md`
+- Modify: `backend/README.md`
+
+**Step 1: Update dependency**
+
+Add `langfuse` to the harness dependencies.
+
+**Step 2: Update docs**
+
+Document:
+- Langfuse environment variables
+- dual-provider behavior
+- failure behavior for explicitly enabled providers
+
+**Step 3: Run targeted verification**
+
+Run: `cd backend && uv run pytest tests/test_tracing_config.py tests/test_model_factory.py tests/test_tracing_factory.py -q`
+Expected: PASS
+
+### Task 4: Run broader regression checks
+
+**Files:**
+- No code changes required
+
+**Step 1: Run relevant suite**
+
+Run: `cd backend && uv run pytest tests/test_tracing_config.py tests/test_model_factory.py tests/test_tracing_factory.py -q`
+
+**Step 2: Run lint if needed**
+
+Run: `cd backend && uv run ruff check packages/harness/deerflow/config/tracing_config.py packages/harness/deerflow/models/factory.py packages/harness/deerflow/tracing`
+
+**Step 3: Review diff**
+
+Run: `git diff -- backend/packages/harness backend/tests README.md backend/README.md`