Harden OSS handoff: maintenance notes, issue intake, and stale policy

[im-sham]

Summary

• add maintenance-mode and compatibility notes to README.md
• add maintenance expectation section to CONTRIBUTING.md
• harden issue intake by requiring nightshift doctor output and explicit environment/version details in bug reports
• add issue template config to disable blank issues and route private security reports
• add SECURITY.md with private vulnerability reporting guidance and best-effort response expectations
• add scheduled stale workflow to auto-manage inactive issues/PRs

Validation

• .venv/bin/python -m pytest -q
• YAML parse check for new/updated template/workflow files