docs: add rules for using safeWriteJson

Add concise rules for using safeWriteJson instead of JSON.stringify with file operations to ensure atomic writes and prevent data corruption.

Signed-off-by: Eric Wheeler <[email protected]>

Author: Eric Wheeler

.roo/rules-code/use-safeWriteJson.md

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+# JSON File Writing Must Be Atomic
+
+- MUST ALWAYS use `safeWriteJson(filePath: string, data: any): Promise<void>` from `src/utils/safeWriteJson.ts` instead of `JSON.stringify` with file-write operations
+- `safeWriteJson` prevents data corruption via atomic writes with locking and streams the write to minimize memory footprint
+- Test files are exempt from this rule