refactor: replace deprecated `ioutil` with `io`/`os` equivalents

[honeynil]

Changes

• Replaced all deprecated ioutil functions with their io or os equivalents (supported since Go 1.16+).
• Removed unused io/ioutil imports.
• Verified that tests pass (go test ./...).

Motivation

The ioutil package is deprecated as of Go 1.16 (see Go 1.16 release notes). This change follows the official migration guidance to modernize the codebase.

Testing

• All existing tests pass.
• Manually verified the changes in a sample project.

Notes

This is a backward-compatible change since io/os equivalents exist in all supported Go versions.

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