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/skills/public/web-design-guidelines/SKILL.md

@@ -0,0 +1,39 @@
+---
+name: web-design-guidelines
+description: Review UI code for Web Interface Guidelines compliance. Use when asked to "review my UI", "check accessibility", "audit design", "review UX", or "check my site against best practices".
+metadata:
+  author: vercel
+  version: "1.0.0"
+  argument-hint: <file-or-pattern>
+---
+
+# Web Interface Guidelines
+
+Review files for compliance with Web Interface Guidelines.
+
+## How It Works
+
+1. Fetch the latest guidelines from the source URL below
+2. Read the specified files (or prompt user for files/pattern)
+3. Check against all rules in the fetched guidelines
+4. Output findings in the terse `file:line` format
+
+## Guidelines Source
+
+Fetch fresh guidelines before each review:
+
+```
+https://raw.githubusercontent.com/vercel-labs/web-interface-guidelines/main/command.md
+```
+
+Use WebFetch to retrieve the latest rules. The fetched content contains all the rules and output format instructions.
+
+## Usage
+
+When a user provides a file or pattern argument:
+1. Fetch guidelines from the source URL above
+2. Read the specified files
+3. Apply all rules from the fetched guidelines
+4. Output findings using the format specified in the guidelines
+
+If no files specified, ask the user which files to review.