feat: MCP tool client infrastructure for agent extensibility

Add the full MCP tool pipeline enabling agents to invoke external tools
(like Brave Search) via MCP servers:

- Add ToolRequest/ToolResponse types and mcp-tool topics to @trustgraph/base
- Create McpToolService (FlowProcessor) that connects to external MCP servers
  via @modelcontextprotocol/sdk StreamableHTTP transport
- Add createMcpTool() to wire MCP tools into the agent's ReAct loop
- Implement config-driven tool registration in AgentService with backward-
  compatible fallback to hardcoded tools
- Add tool filtering by group and state (port of Python tool_filter.py)
- Register mcp-tool in gateway dispatcher and export from @trustgraph/flow
- Fix flow restart race condition: skip restart when flow definitions unchanged
- Update seed config with MCP server config and tool definitions
- Add run scripts for MCP tool service and Brave Search MCP server

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ts/packages/flow/src/gateway/dispatch/manager.ts

@@ -31,6 +31,7 @@ const FLOW_SERVICES: ReadonlyMap<string, { request: string; response: string }>
   ["graph-embeddings",    { request: "graph-embeddings-request",    response: "graph-embeddings-response" }],
   ["document-embeddings", { request: "doc-embeddings-request",      response: "doc-embeddings-response" }],
   ["triples",             { request: "triples-request",             response: "triples-response" }],
+  ["mcp-tool",            { request: "mcp-tool-request",            response: "mcp-tool-response" }],
 ]);
 
 /**