feat: epics, subtasks and checklists

Tickets can hang from another one of the same project, three levels deep
at most (epic → ticket → subtask). The ticket page lists its subtasks with
their progress, creates new ones and moves the ticket under an epic; a new
Épicas tab shows every epic and how far it has got. Each ticket also gets a
checklist for the small steps that don't deserve a ticket.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/domain/src/index.ts

@@ -4,3 +4,4 @@ export * from './lib/models/inputs';
 export * from './lib/models/workflow';
 export * from './lib/ports/gateways';
 export * from './lib/models/mentions';
+export * from './lib/models/hierarchy';