chore: refresh docs and workflows

[spiegel-im-spiegel]

Summary

• add .github/copilot-instructions.md with project guidance (including errs-based error handling)
• restructure and expand README.md to align with errs-style chapter layout and clarify local task usage
• reorganize GitHub Actions workflows into ci and CodeQL
• remove legacy workflow files (lint.yml, vulns.yml, codeql-analysis.yml)
• bump github.com/goark/errs from v1.3.3 to v1.3.4

Validation

• task test

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