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/README.md

@@ -0,0 +1,55 @@
+# Documentation
+
+This directory contains detailed documentation for the DeerFlow backend.
+
+## Quick Links
+
+| Document | Description |
+|----------|-------------|
+| [ARCHITECTURE.md](ARCHITECTURE.md) | System architecture overview |
+| [API.md](API.md) | Complete API reference |
+| [CONFIGURATION.md](CONFIGURATION.md) | Configuration options |
+| [SETUP.md](SETUP.md) | Quick setup guide |
+
+## Feature Documentation
+
+| Document | Description |
+|----------|-------------|
+| [STREAMING.md](STREAMING.md) | Token-level streaming design: Gateway vs DeerFlowClient paths, `stream_mode` semantics, per-id dedup |
+| [FILE_UPLOAD.md](FILE_UPLOAD.md) | File upload functionality |
+| [PATH_EXAMPLES.md](PATH_EXAMPLES.md) | Path types and usage examples |
+| [summarization.md](summarization.md) | Context summarization feature |
+| [plan_mode_usage.md](plan_mode_usage.md) | Plan mode with TodoList |
+| [AUTO_TITLE_GENERATION.md](AUTO_TITLE_GENERATION.md) | Automatic title generation |
+
+## Development
+
+| Document | Description |
+|----------|-------------|
+| [TODO.md](TODO.md) | Planned features and known issues |
+
+## Getting Started
+
+1. **New to DeerFlow?** Start with [SETUP.md](SETUP.md) for quick installation
+2. **Configuring the system?** See [CONFIGURATION.md](CONFIGURATION.md)
+3. **Understanding the architecture?** Read [ARCHITECTURE.md](ARCHITECTURE.md)
+4. **Building integrations?** Check [API.md](API.md) for API reference
+
+## Document Organization
+
+```
+docs/
+├── README.md                  # This file
+├── ARCHITECTURE.md            # System architecture
+├── API.md                     # API reference
+├── CONFIGURATION.md           # Configuration guide
+├── SETUP.md                   # Setup instructions
+├── FILE_UPLOAD.md             # File upload feature
+├── PATH_EXAMPLES.md           # Path usage examples
+├── summarization.md           # Summarization feature
+├── plan_mode_usage.md         # Plan mode feature
+├── STREAMING.md               # Token-level streaming design
+├── AUTO_TITLE_GENERATION.md   # Title generation
+├── TITLE_GENERATION_IMPLEMENTATION.md  # Title implementation details
+└── TODO.md                    # Roadmap and issues
+```