feat: replace Jina REST with MCP tools for web search
- Add langchain-mcp-adapters dependency
- New app/tools/mcp_jina.py: Jina MCP client with filtered tool loading
(search_web, read_url, sort_by_relevance)
- Update resolve_tools to return (tools, needs_jina_mcp) tuple
- Update builder.py: skip graph cache for MCP-bound tools, add search
tool guidance to system prompt
- Refactor chat.py: extract _run_graph_stream, add MCP/non-MCP paths,
update tool titles and summaries for MCP tool names
- Tested locally: both MCP search path and non-MCP knowledge path work
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
