Addresses state safety issues found during #1062 deep dive:
1. completed-units.json writes in auto-worktree.ts and auto-worktree-sync.ts
used plain writeFileSync which could produce truncated/corrupt files on
crash, losing completion keys and causing unit re-dispatch. Switched to
atomicWriteSync (temp file + rename) for crash safety.
2. Plan file checkbox reconciliation in auto-worktree.ts also switched to
atomicWriteSync to prevent partial PLAN.md writes on crash.
3. db-writer.ts functions (saveDecisionToDb, updateRequirementInDb,
saveArtifactToDb) wrote markdown files via saveFile() without invalidating
caches afterward. Added targeted cache invalidation (state + path + parse)
so deriveState() always sees fresh data. Uses individual invalidation
functions rather than invalidateAllCaches() to avoid clearing the artifacts
table that was just written to.
60dfaabe03