A 'Customer note' field on ENG issues for the release notes draft

The "Release notes draft" agent task in Emargen reads the ENG issues closed since its last run. Most of them don't say what changes for a customer, so Leo rewrites every line.

• New field "Customer note": one plain sentence saying what changes for customers, or empty if nothing does.
• The release process page asks for it on anything customers can see (Confluence ENG › Release process).