ci: require a #openab-pty Discord discussion link on community PRs

[chaodu-agent]

Same rule as openabdev/openab (pr-discussion-check.yml + close-stale-prs.yml):

• When a community PR body has no Discord discussion URL, it gets the closing-soon label and a comment asking for a link to the #openab-pty discussion. The check fails, and the PR is auto-closed 24 h after labelling. Adding the link removes both the label and the comment.
• Bots and maintainers (write/maintain/admin on this repo) are exempt.

Differences from openab, because this repo lacks what they need:

• GITHUB_TOKEN instead of the GitHub App token, since there is no APP_ID/APP_PRIVATE_KEY here.
• The maintainer exemption checks repo permission instead of openabdev/openab-maintainers team membership, since no team is attached and GITHUB_TOKEN cannot read org teams.

pull_request_target is safe here: nothing from the PR is checked out or executed, and the script only reads the PR body and author from the event.

CHANNEL_URL is empty for now, so any discord.com/channels/… link passes. Set it to the #openab-pty channel link to make the comment clickable and accept only links into that channel. Matching logic checked against 6 cases: no link, any link, a message in the channel, another channel, the discordapp domain, an invite link.

The closing-soon label was created in this repo.