Plugin marketplace proxy and Slack reaction tools

Features

• Marketplace proxy for private plugin installs #2614 - Adds a stateless marketplace proxy that streams plugins from GitHub through token-gated routes, including a Claude Code marketplace manifest endpoint and a git Smart HTTP proxy for plugin source clones. The Plugins page surfaces install instructions for Claude Code, Claude Cowork, and Cursor in a new modal. (Author: @bradcypert)
• Authorization challenge management API #2604 - Adds listChallenges and resolveChallenge endpoints to the access service, powering the challenge resolution UI. (Author: @adaam2)
• Slack reaction platform tools #2628 - Adds platform_slack_add_reaction, platform_slack_remove_reaction, platform_slack_get_reactions, platform_slack_list_reactions, and platform_slack_list_emoji so assistants can react to messages and discover available emoji. (Author: @danielkov)
• Default-attach Slack reaction tools in onboarding #2631 - Default-attaches Slack reaction tools during assistant onboarding and injects reaction etiquette guidance into the assistant's behavior section. The Slack manifest builder now maps the reaction handlers to the reactions:write, reactions:read, and emoji:read bot scopes. (Author: @danielkov)
• Custom risk policy messages #2533 - Allows setting custom policy messages that are shown to end users when a risk policy fires. (Author: @chase-crumbaugh)
• Publish observability-only plugins #2610 - Allows publishing to GitHub when the org has only the observability plugin and no custom plugins. (Author: @bradcypert)