Deduplicate Slack CI notifications with artifact-based state tracking (#63676)

Slack notifications for CI failures and missing doc inventories were
posted on every failing run regardless of whether the failure was
already reported. This adds per-branch state tracking via GitHub
Actions artifacts so notifications are only sent when the set of
failures changes or 24 hours pass (as a "still not fixed" reminder).
Recovery notifications are posted when a previously-failing run passes.