feat(core): update formatting of agent rules documentation

Markdown documents should only have one h1 header, it is pretty unlikely that an existing agents config does not already have one, switching to h2 instead should make most linters happy.

In theory you could make this a little smarter by checking beforehand..

Author: Stanzilla

packages/nx/src/ai/set-up-ai-agents/get-agent-rules.ts

@@ -1,6 +1,6 @@
 export function getAgentRules(nxCloud: boolean) {
   return `
-# General Guidelines for working with Nx
+## General Guidelines for working with Nx
 
 - When running tasks (for example build, lint, test, e2e, etc.), always prefer running the task through \`nx\` (i.e. \`nx run\`, \`nx run-many\`, \`nx affected\`) instead of using the underlying tooling directly
 - You have access to the Nx MCP server and its tools, use them to help the user