feat: improve issue reminder workflow and introduce conditional autoclose (#14477)

* feat: improve issue reminder workflow and introduce conditional auto-close

* up

* filter the comments coming from the bot.

* include older PRs.

* implement point 1 of Yiyi's feedback

* clarify reminder message

* report each at-risk PR to Slack once, widen the rescue window

The Slack rescue branch fired on every daily run between SLACK_RESCUE_DAYS and
AUTOCLOSE_DAYS, so a PR the team decided not to rescue came back three times.
Append a hidden marker to the warning comment once a PR has been reported, and
skip PRs that already carry it.

Also move the Slack ping from 7 to 5 days after the warning, leaving 5 days
(instead of 3) between the message and the auto-close, so a Friday message
still gets a reply.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* clarify reminder message

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>