Fixes#3164
Validators and find-phase hardcoded phasesDir = .planning/phases/, so projects
using the milestone-archive layout (.planning/milestones/v*-phases/) had an empty
diskPhases set, triggering W006 for every active phase and find-phase returning
found:false.
Add collectDiskPhases(planBase) helper that scans both flat layout and all
.planning/milestones/v*-phases/ subdirs. Wire it into cmdValidateConsistency,
cmdValidateHealth (both the Check 4 validPhases set and Check 8 diskPhases),
and refactor cmdFindPhase to iterate candidate search dirs so it also searches
milestone-archive dirs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>