🤖 fix: use atomic writes for partial.json to prevent corruption

Non-atomic fs.writeFile can leave truncated/malformed JSON if the app
crashes mid-write. This causes 'Unexpected end of JSON input' errors
when reading partial.json on the next stream start, leading to amnesia
(messages being lost).

Use write-file-atomic which writes to a temp file then renames,
ensuring readers always see either the old complete file or the new
complete file, never a partial write.

Fixes #803

Author: ammar-agent

src/node/services/partialService.ts

@@ -1,5 +1,6 @@
 import * as fs from "fs/promises";
 import * as path from "path";
+import writeFileAtomic from "write-file-atomic";
 import type { Result } from "@/common/types/result";
 import { Ok, Err } from "@/common/types/result";
 import type { MuxMessage } from "@/common/types/message";
@@ -80,7 +81,9 @@ export class PartialService {
           },
         };
 
-        await fs.writeFile(partialPath, JSON.stringify(partialMessage, null, 2));
+        // Atomic write: writes to temp file then renames, preventing corruption
+        // if app crashes mid-write (prevents "Unexpected end of JSON input" on read)
+        await writeFileAtomic(partialPath, JSON.stringify(partialMessage, null, 2));
         return Ok(undefined);
       } catch (error) {
         const message = error instanceof Error ? error.message : String(error);