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/scripts/run-mcp-tool.ts

@@ -0,0 +1,18 @@
+/**
+ * Start the MCP tool service.
+ *
+ * Usage: pnpm tsx scripts/run-mcp-tool.ts
+ *
+ * Env:
+ *   NATS_URL (default: nats://localhost:4222)
+ */
+import { McpToolService } from "../packages/flow/src/agent/mcp-tool/index.js";
+
+async function run(): Promise<void> {
+  await McpToolService.launch("mcp-tool");
+}
+
+run().catch((err) => {
+  console.error("MCP tool service failed:", err);
+  process.exit(1);
+});