feat: adopt Go 1.25 enhancements

[hyp3rd]

Summary

• streamline metadata copying with maps.Clone
• add ErrorGroup.Join plus slog logging adapter
• document modern cloning and logging options

Testing

• go test $(go list ./... | grep -v '/test$')

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https://chatgpt.com/codex/tasks/task_e_68a43a8e4c7c8330bddc3f25e1e63248

[Copilot]

Pull Request Overview

This PR adopts Go 1.25 enhancements to modernize the codebase by leveraging new standard library features for improved performance and developer experience.

• Replace manual metadata copying with maps.Clone for cleaner code
• Add ErrorGroup.Join method using errors.Join and modernize error cloning with slices.Clone
• Introduce slog logging adapter with comprehensive documentation

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
errors.go	Replaces manual metadata copying with maps.Clone for cleaner code
error_group.go	Adds Join() method using errors.Join and modernizes Errors() with slices.Clone
error_group_test.go	Adds test coverage for the new Join() method
pkg/ewrap/adapters/slog.go	Implements new slog logging adapter for Go 1.21+
pkg/ewrap/adapters/slog_test.go	Provides test coverage for the slog adapter
docs/docs/features/logging.md	Documents the new slog integration
docs/docs/features/error-groups.md	Updates documentation to recommend Join() over legacy methods
README.md	Updates examples to use new Join() method and documents slog support

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