[Bug] addSession does no I/O error path for the legacy migration race

[akramcodez]

Description

If two processes start simultaneously and both decide to migrate usage.json, the second renameSync raises EEXIST and the fallback copy-overwrites the freshly-renamed file. The user data ends up being whatever the second process wrote.

Environment

• OS: Any
• Node version: v22.20.0
• Nanocoder version: 1.30.0
• Provider: N/A
• Model: N/A

Steps to Reproduce

1. Trigger a legacy usage migration from two concurrent instances.

Expected Behavior

Needs a rename no-clobber or fsync-and-swap pattern to handle the migration safely.

Actual Behavior

The second process overwrites the file, potentially causing data loss.

Additional Context

• I have searched existing issues to ensure this is not a duplicate
• I can reproduce this issue consistently
• This issue occurs with the latest version of nanocoder