cmdPhaseComplete determines is_last_phase and next_phase by scanning
.planning/phases/ directories on disk. Phases defined in ROADMAP.md
but not yet planned (no directory created) are invisible to this scan,
causing premature is_last_phase:true when only the first phase has
been scaffolded.
Add a fallback that parses ROADMAP.md phase headings when the
filesystem scan finds no next phase. Uses the existing comparePhaseNum
utility for consistent ordering with letter suffixes and decimals.
Closes#709
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>