build-in-public: stream releases + build updates to the WardenBot community Slack/Discord

[pardamike]

As WardenBot launches community Slack + Discord, post pytest-wardenbot release / build / CI updates to those channels so people can follow development in the open ("build in public").

• Future version (post-v0.2); not part of the v0.2 roadmap.
• Likely a GitHub Action posting on release/tag (and maybe CI milestones) to a community webhook.
• Distinct from feat(v0.2): bundled Slack + Discord chat-platform adapters ([slack] / [discord] extras) #7 (the Slack/Discord test adapters) — this is about our own community channels, not testing user bots.
• Split into concrete tasks once the channels exist and the format firms up.

Logged as a side item per maintainer aside.