Replaces direct fs.writeFileSync calls for STATE.md, ROADMAP.md, and
config.json with write-to-temp-then-rename so a process killed mid-write
cannot leave an unparseable truncated file. Falls back to direct write if
rename fails (e.g. cross-device). Adds regression tests for the helper.
Closes#1915
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
