Three-pillar integration taxonomy (Datasource / Chat / Action) + unified install pipeline. workspace_plugins becomes the universal install record; the connections table drops in a one-shot migration (slice 6, #2744). Folds in the 7 non-Slack chat install models that were placeholder catalog entries after 1.5.2 — Telegram / Discord / Linear / GitHub / Teams / WhatsApp / Google Chat — covering the three install model shapes (OAuth, Form/StaticBot, Service-account). See PRD #2738, ADR-0006, and ADR-0007. Supersedes #2662.