Rename `no_memories_if_mcp_or_web_search` →
`disable_on_external_context` with backward compatibility
While doing so, we add a key alias system on our layer merging system.
What we try to avoid is a case where a company managed config use an old
name while the user has a new name in it's local config (which would
make the deserialization fail)
cfc23eee3d