Custom team roles: removed CHECK constraint on team_members.role, roles stored in teams.settings, roles API, stage_config.required_role enforced on claim. Multi-party signoff: workflow_signoffs table, validation gate in advanceInternal, SubmitSignoff engine method, signoff HTTP API. Frontend: dynamic role management, stage config validation UI, signoff panel. Design docs for extension lifecycle and trigger composition. 20 store tests passing. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>