Update Copilot release instructions and reorganize workflows

[spiegel-im-spiegel]

Summary

• add a release process section to .github/copilot-instructions.md
• reorganize GitHub Actions workflows to match the fetch repository style
  • add .github/workflows/ci.yml (lint/test + govulncheck)
  • add .github/workflows/codeql.yml (CodeQL v3)
  • remove legacy lint.yml, vulns.yml, and codeql-analysis.yml

Notes

• branch includes two commits:
  • 74cc85a Add release process to copilot instructions
  • ca47f00 Reorganize GitHub workflows to fetch-style CI

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