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/.gitignore

@@ -0,0 +1,59 @@
+# DeerFlow docker image cache
+docker/.cache/
+# oh-my-claudecode state
+.omc/
+# OS generated files
+.DS_Store
+*.local
+._*
+.Spotlight-V100
+.Trashes
+ehthumbs.db
+Thumbs.db
+
+# Python cache
+__pycache__/
+*.pyc
+*.pyo
+
+# Virtual environments
+.venv
+venv/
+
+# Environment variables
+.env
+
+# Configuration files
+config.yaml
+mcp_config.json
+extensions_config.json
+
+# IDE
+.idea/
+.vscode/
+
+# Coverage report
+coverage.xml
+coverage/
+.deer-flow/
+.claude/
+skills/custom/*
+logs/
+log/
+
+# Local git hooks (keep only on this machine, do not push)
+.githooks/
+
+# pnpm
+.pnpm-store
+sandbox_image_cache.tar
+
+# ignore the legacy `web` folder
+web/
+
+# Deployment artifacts
+backend/Dockerfile.langgraph
+config.yaml.bak
+.playwright-mcp
+.gstack/
+.worktrees