ci: enforce single-flight golden path bootstrap runs

[dmoliveira]

Summary

• Change Golden Path Bootstrap concurrency key to repository-level single-flight so new runs cancel older in-progress runs even across refs.
• Keep cancellation enabled to reduce queued duplicate bootstrap jobs.
• Clarify single-flight behavior in README maintenance notes.

Validation

• pre-commit run check-yaml --all-files
• pre-commit run markdownlint-cli2 --all-files
• make ci-quick
• uvx ruff check .