Plugin marketplace proxy and Slack reaction tools

This release ships a token-gated marketplace proxy that lets Claude Code, Claude Cowork, and Cursor install Gram-published plugins without exposing the upstream GitHub repo. It also adds Slack reaction platform tools, custom policy messages, and authorization challenge management endpoints.

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 )