fix(slack): fix disappearing tags on issue assignment

[cathteng]

Actually persist the tags when an issue is assigned.

The modal actions (resolve/archive) will require another pass, because there are two calls to the webhook endpoint (open the modal, taken action via the modal) and we need to modify the private_metadata that is being passed back so we can extract the tags from the payload.

Fix

https://github.com/user-attachments/assets/d543603a-685a-4793-b943-e6b766f0c15e

Before

https://github.com/user-attachments/assets/9b01534b-ae84-420b-aa5a-17c4193bcd3d