cmdMilestoneComplete hardcoded three archive paths to root .planning/
while its siblings (roadmap/requirements/state/phases) used workstream-aware
planningPaths. So `milestone complete <v> --ws <name>` scattered its archive
into root and never created workstream-local milestones/.
Derive the archive base from the workstream-aware planning root:
- milestonesPath / archiveDir / auditFile now use planningPaths(cwd).planning
- flat mode (no --ws) is a no-op (planningPaths(cwd).planning == root .planning)
Regression in tests/milestone.test.cjs: --ws archives into the workstream
milestones dir, not root.
Closes#1911
