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/docs/MEMORY_IMPROVEMENTS_SUMMARY.md

@@ -0,0 +1,38 @@
+# Memory System Improvements - Summary
+
+## Sync Note (2026-03-10)
+
+This summary is synchronized with the `main` branch implementation.
+TF-IDF/context-aware retrieval is **planned**, not merged yet.
+
+## Implemented
+
+- Accurate token counting with `tiktoken` in memory injection.
+- Facts are injected into `<memory>` prompt content.
+- Facts are ordered by confidence and bounded by `max_injection_tokens`.
+
+## Planned (Not Yet Merged)
+
+- TF-IDF cosine similarity recall based on recent conversation context.
+- `current_context` parameter for `format_memory_for_injection`.
+- Weighted ranking (`similarity` + `confidence`).
+- Runtime extraction/injection flow for context-aware fact selection.
+
+## Why This Sync Was Needed
+
+Earlier docs described TF-IDF behavior as already implemented, which did not match code in `main`.
+This mismatch is tracked in issue `#1059`.
+
+## Current API Shape
+
+```python
+def format_memory_for_injection(memory_data: dict[str, Any], max_tokens: int = 2000) -> str:
+```
+
+No `current_context` argument is currently available in `main`.
+
+## Verification Pointers
+
+- Implementation: `packages/harness/deerflow/agents/memory/prompt.py`
+- Prompt assembly: `packages/harness/deerflow/agents/lead_agent/prompt.py`
+- Regression tests: `backend/tests/test_memory_prompt_injection.py`