Fix worker commit message format for gitlint

In Worker.hs, post-process amp output to create gitlint-compliant commit messages:

• First line of amp output becomes the subject
• Blank line separator
• Rest of output becomes the body
• Task-Id trailer at the end

Example: Subject line here

Rest of the description goes in the body.

Task-Id: t-xxxxxxxx

This ensures gitlint's B4 rule (second line must be empty) is satisfied.