Fix broken MCP path references after server move

[yevheniikravchuk]

Why

There is a stale MCP path in this repo’s docs that still points at products/mcp, which is no longer the active location in the PostHog repo.

PostHog moved MCP to services/mcp, and this update keeps links accurate for users who follow the README/agent docs.

Historical context (verified)

• Root cause is PostHog commit c4fe76bdcb85b7b0c6c44079a34854aa538c5005 dated 2025-12-23
  (commit message: chore(mcp): move mcp server to services folder (#43928)).
• That commit is the MCP migration PR: chore(mcp): move mcp server to services folder posthog#43928.
• The migration renamed the folder from products/mcp to services/mcp, so the old path now 404s and we should point users at the new path.

What changed

• Updated the broken MCP link in README.md to https://github.com/PostHog/posthog/tree/master/services/mcp.
• Updated the same MCP path in AGENTS.md for consistency across repo guidance.

Evidence

• https://github.com/PostHog/posthog/tree/master/products/mcp now returns 404.
• https://github.com/PostHog/posthog/tree/master/services/mcp is reachable.

Human summary

This is a small, low-risk docs-only fix to prevent dead links after the MCP folder migration.

Testing

• Not run (documentation-only change).
• By virtue of opening said links in the browser, like men of Victorian culture used to do back in the day.