Add stale workflow for 180-day inactive issues and PRs (#1594)

* Initial plan

* Add stale issues and PRs workflow

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>

* Add 7-day grace period before closing stale items

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>

* Update stale timing: mark at 150 days, close at 180 days

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>