Produce a themed Slack post announcing Ark releases. Pulls from BOTH the Ark core repo (mckinsey/agents-at-scale-ark) and the Marketplace repo (mckinsey/agents-at-scale-marketplace), validates docs links, groups features by theme with relevant emoji, and ends with a "Coming next" section sourced from the current sprint on the ARK project board. Invoke when the user asks for a "release announcement", "slack post", "what's new since vX", "changelog for