fix: preserve user env in settings.json

[istarwyh]

Summary

• preserve user-defined env entries in ~/.claude/settings.json when ccc runs
• continue removing provider-specific and Claude/Anthropic-prefixed env values from persisted settings
• update provider tests to cover the preserved user env behavior

Testing

• go test ./internal/provider ./internal/config

Related

• fixes ccc removes user-defined env field from ~/.claude/settings.json on every run #74

[guyskk]

Closed in favor of #76 which implements a more robust solution: properly distinguishes user env from ccc-managed env, filters user env for settings.json, and only passes base+provider env to subprocess.