Fix dangling doc links and add cookiecutter generation tests

• Fixed dangling links across README.md, MIGRATION-GUIDE.md, and .agent/rules.md left over from the Confluence migration (02408660) — several pointed at CONTRIBUTING.md anchors that no longer exist. GitLab-settings and changelog references now point at the Confluence "OPS: Code" page instead.
• Removed a dead CONTRIBUTING.md "Setup" link — prek installs automatically via uv sync, no separate step needed.
• Cleaned up CHANGELOG.md: replaced the stale "openBIS marked not-yet-usable" entries with a single accurate "Removed" entry now that the integration has been deleted entirely.
• Added tests/test_template_generation.py, which drives cookiecutter to generate both a standard and a data_project repo and runs ruff/mypy/pytest inside each — verifying the template actually produces a working project, not just that generation doesn't error.
• Re-enabled the pytest CI job (previously disabled) to run this new suite; it now installs git since the generated project's post-gen hook needs it.
• Added mypy as a local pre-push hook (mirroring the blueprint's own pattern) instead of a CI job — keeps the strict/slow check on contributors' machines rather than the shared GEG runner.