ensure changelog uses softwraps

chatted briefly with @zanieb and it seemes the github release notes use a different renderer than the code renderer (is this new???) and so their changelog was displayed with hard wraps. since we are processing that string anyways, we could/should update it to make sure it will display properly in github.

Trace where cargo-dist processes the changelog string and where GitHub release notes are generated. Reproduce the hard-wrap rendering in GitHub, then verify the processed changelog displays with soft wraps there. The issue names no files or tests, so locating the relevant release-notes path is the first step.

Written by the indexing model from the issue text.