cmdMilestoneComplete previously appended new milestone entries at the
end of MILESTONES.md. Over successive milestones this produced
chronological order (oldest first), which is counterintuitive — users
expect the most recent milestone at the top.
This fix detects the file header (h1-h3) and inserts the new entry
immediately after it, pushing older entries down. Files without a
recognizable header get the entry prepended. New files still get a
default '# Milestones' header.
Adds 2 tests: single insertion ordering assertion and three-sequential-
completions verification (v1.2 < v1.1 < v1.0 in file order).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
732ea09f81