fix(config): honor false boolean env overrides

[huklaa]

Summary

• Make "DRY_RUN" and "APPROVAL_MODE" override file configuration whenever the environment variable is present.
• Preserve file/default values when these environment variables are absent.
• Add regression coverage for overriding file-provided "true" values with environment-provided "false".

Problem

The existing "os.Getenv" checks only assign these fields when the environment value is "true" or "1". As a result, an explicitly configured "false" is treated like an absent variable and cannot override a "true" value loaded from the JSON configuration file.

This change uses "os.LookupEnv", so variable presence determines precedence while its parsed value determines the resulting boolean.

Validation

• "go test ./..."
• "go test -race ./internal/config"
• "go vet ./..."
• "git diff --check"

A final duplicate/WIP check found no related issue, pull request, branch, or recent commit.