Add Slack Channel

Adds Slack support via the Chat SDK bridge. Trunk ships no channels — this skill copies the Slack channel layer (adapter, shared lib, bot-inbound guard, provisioning core, container skills) in from the channels branch. The Apply steps carry nc: directive fences (an agent applies the prose, a parser the directives); all idempotent.

This is the base Slack experience: one bot, DM and channel chat. The Slack agents feature — child bots provisioned from create_agent, shared rooms, canvases, DM onboarding — ships separately in /slack-a2a-rooms + /slack-agent-flow, in-tree under .claude/skills/ (their canonical home; the channels branch keeps a compatibility copy for older checkouts). The setup wizard applies them automatically, and they can be applied on top of this install at any time. Existing classic installs that want the Slack agents experience should use /migrate-slack-agents rather than re-running this skill (classic keeps working; that migration is optional).