fix: use atomic writes in FileExporter.export() to prevent corrupted trace files (#646)

`FileExporter.export()` used `fs.writeFile` directly, while the snapshot mechanism
used atomic writes (tmp + rename). When concurrent sessions wrote to the same trace
file, a snapshot rename could race with a non-atomic writeFile, producing truncated
JSON ("Unterminated string" parse error).

Also adds temp file cleanup on failure, matching the existing `snapshot()` pattern.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: anandgupta42

packages/opencode/src/altimate/observability/tracing.ts

@@ -172,19 +172,25 @@ export class FileExporter implements TraceExporter {
   }
 
   async export(trace: TraceFile): Promise<string | undefined> {
+    let tmpPath: string | undefined
     try {
       await fs.mkdir(this.dir, { recursive: true })
       // Sanitize sessionId for safe file name (defense-in-depth — also sanitized in Trace)
       const safeId = (trace.sessionId ?? "unknown").replace(/[/\\.:]/g, "_") || "unknown"
       const filePath = path.join(this.dir, `${safeId}.json`)
-      await fs.writeFile(filePath, JSON.stringify(trace, null, 2))
+      // Atomic write: write to temp file, then rename — prevents partial reads
+      // when concurrent snapshots or exports target the same file
+      tmpPath = filePath + `.tmp.${Date.now()}.${Math.random().toString(36).slice(2, 8)}`
+      await fs.writeFile(tmpPath, JSON.stringify(trace, null, 2))
+      await fs.rename(tmpPath, filePath)
 
       if (this.maxFiles > 0) {
         this.pruneOldTraces().catch(() => {})
       }
 
       return filePath
     } catch {
+      if (tmpPath) fs.unlink(tmpPath).catch(() => {})
       return undefined
     }
   }