Replace 11 fs.writeFileSync calls with atomicWriteFileSync in three
files that write to .planning/ artifacts (ROADMAP.md, REQUIREMENTS.md,
MILESTONES.md, and frontmatter updates). This prevents partial writes
from corrupting planning files on crash or power loss.
Skipped low-risk writes: .gitkeep (empty files) and archive directory
writes (new files, not read-modify-write).
Files changed:
- milestone.cjs: 5 sites (REQUIREMENTS.md, MILESTONES.md)
- phase.cjs: 5 sites (ROADMAP.md, REQUIREMENTS.md)
- frontmatter.cjs: 2 sites (arbitrary .planning/ files)
Closes#1972
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>