## Summary
- add an experimental guide for registering customer-hosted MCP servers
- document supported authentication, the registration and sync flow,
toolkit behavior, and customer-facing workarounds for known gaps
- add the guide to the Extending sessions navigation and cross-link it
from the sessions-over-MCP guide
## Why
Custom MCP toolkits now have a working platform lifecycle, but customers
need one place that clearly distinguishes remote MCP servers from
in-process custom tools. The guide also makes the current pilot
limitations explicit so customers know which API version, sync flow,
account selection, and hosting model to use.
## Validation
- `cd docs && bun run lint:links`
- `cd docs && bun run types:check`
- `git diff --check`
Docs-only change, so no changeset is required.
shams haroon