bug: completed sprint does not close milestone or retain completed issue history

[trsdn]

Problem

A sprint can be fully completed in AiScrum, but the GitHub milestone and dashboard completion state do not stay in sync.

Concrete example: Sprint 235 appears to have finished all 3 issues successfully, but:

• the sprint still appears open in GitHub
• the completed issues do not show up under the closed milestone view as expected
• the dashboard/header completion indicator is not green even though the sprint is done

That makes it look like the sprint never closed properly, even though the work itself completed.

Expected behavior

When a sprint is truly complete, all sprint-completion surfaces should agree:

• dashboard shows a completed/green sprint state
• GitHub milestone is closed
• completed sprint issues remain correctly associated with that sprint milestone for historical review

Acceptance criteria

• When all sprint issues are completed successfully, the sprint is finalized as complete in the dashboard/UI.
• The completion indicator/logo becomes green for a completed sprint.
• The corresponding GitHub sprint milestone is closed automatically.
• Completed issues remain associated with the sprint milestone so they are visible in the milestone's closed/completed view.
• Sprint completion logic does not remove or lose milestone association for completed issues.
• Regression coverage is added for sprint-finalization, milestone closing, and issue-milestone retention.
• Use Sprint 235 as a concrete verification case or regression fixture if possible.