cmdStateBeginPhase replaced the entire ## Current Position section with only
Phase and Plan lines, destroying Status, Last activity, and Progress fields.
cmdStateAdvancePlan then failed to update these fields since they no longer
existed.
Now begin-phase updates individual lines within Current Position instead of
replacing the whole section. Also adds updateCurrentPositionFields helper so
advance-plan keeps the Current Position body in sync with bold frontmatter
fields.
Fixes#1365
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
43d1787670