config.json Save() should use atomic write pattern

## Context
`config.json` writes use direct `os.WriteFile` which can produce torn reads if the process is interrupted mid-write. The environment package already has `WriteFileAtomic` (write-to-temp + rename) for `.env` files.

## Proposal
Apply the same atomic write pattern to `config.json` saves. Depends on #WriteFileAtomic being available in a shared location (see companion issue for hoisting to pkg/osutil).

## References
- PR #7776 follow-up from concurrency review