fix(config): serialize user settings writes atomically

[huyua9]

Summary

• write user_config.yaml through a same-directory temporary file and atomic rename
• serialize read-modify-write updates used by settings, editor width, clean-slate, and CLI configuration
• preserve unrelated settings such as editor width during the settings form update

Validation

• go test -race ./internal/config
• go test ./... -run ^
• go vet ./...
• Full go test ./... is blocked by the existing TestHabitFlow expectation for success=done; HEAD 0fe2cd7 reproduces the same failure.

Closes #25