That sounds pretty toxic tbh. Your partner should not control who you can and can not be friends with.

It's not like you cheated on him. I get having boundaries with other gender friends but not having friends of another gender seems a bit much and bound to fail.

Don’t worry about it

That’s tough. Personally, I don’t see the problem because nothing happened, but you did have a rule in place. I would be honest with him. He might get upset, or he might not. If he loves you, he will forgive.

It’s a childish rule. You should both grow up.

Just talking and hugs?

I know someone who has this rule in their relationship as well and they are now married. Been together 10 years. Both are super jealous and so it made sense to have that rule.

I don't think anyone without this rule find it wrong or a big deal. You laid it out as if you had fucked your neighbor behind your partners back but instead it was a few hugs and hanging out in a group where there was men and women.
I personally couldn't care less about some platonic meetings but if your boyfriend is really really jealous (insecure) it might feel like cheating and a betrayal.

But he deserves to know. You can't walk around hiding this from him it's not good for the relationship.

That's a very selfish rule to have, who's the one that imposed it?