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/api/tracker/domain/src/lib/issue/issue.spec.ts

@@ -4,10 +4,12 @@ import {
   ValidationError,
 } from '@task-stream/api/shared/domain';
 import { IssueCreatedEvent } from '../events/issue-created.event';
+import { IssueParentChangedEvent } from '../events/issue-parent-changed.event';
 import { IssueStatusChangedEvent } from '../events/issue-status-changed.event';
 import { IssueFactory } from '../factories/issue.factory';
 import { replay } from '../testing';
 import { Issue } from './issue';
+import { ParentCandidate } from './issue-hierarchy';
 import { mentionedUserIds, withoutMentionMarkup } from './mentions';
 import { IssueLinkType, IssueStatus, IssueType } from './issue.enums';
 
@@ -223,6 +225,93 @@ describe('Issue', () => {
     });
   });
 
+  describe('hierarchy', () => {
+    const parent = (over: Partial<ParentCandidate> = {}): ParentCandidate => ({
+      id: 'epic-1',
+      key: 'TS-1',
+      projectId: 'p-1',
+      ancestorIds: [],
+      ...over,
+    });
+
+    it('hangs from a parent, moves and leaves, and survives a replay', () => {
+      const issue = newIssue();
+      issue.setParent('u-pm', parent());
+      issue.setParent('u-pm', parent());
+      issue.setParent('u-pm', parent({ id: 'epic-2', key: 'TS-2' }));
+      issue.setParent('u-pm', null);
+      issue.setParent('u-pm', parent({ id: 'epic-3', key: 'TS-3' }));
+
+      const changes = issue
+        .getUncommittedEvents()
+        .filter((e) => e instanceof IssueParentChangedEvent);
+      expect(changes.map((e) => [e.parentId, e.previousParentId])).toEqual([
+        ['epic-1', null],
+        ['epic-2', 'epic-1'],
+        [null, 'epic-2'],
+        ['epic-3', null],
+      ]);
+      expect(replay(issue, Issue).rebuilt.parentId).toBe('epic-3');
+    });
+
+    it('refuses itself, another project, loops and a fourth level', () => {
+      const issue = newIssue();
+      expect(() => issue.setParent('u', parent({ id: issue.id }))).toThrow(
+        BusinessRuleError,
+      );
+      expect(() => issue.setParent('u', parent({ projectId: 'p-2' }))).toThrow(
+        /another project/,
+      );
+      expect(() =>
+        issue.setParent('u', parent({ ancestorIds: [issue.id] })),
+      ).toThrow(/loop/);
+      // Under a ticket that hangs from an epic: it becomes a subtask.
+      expect(() =>
+        issue.setParent('u', parent({ ancestorIds: ['t-1'] })),
+      ).not.toThrow();
+      expect(() =>
+        issue.setParent('u', parent({ id: 'x', ancestorIds: ['t-1', 'e-1'] })),
+      ).toThrow(/at most 2 levels/);
+      // A ticket with subtasks cannot go below a ticket that has a parent.
+      expect(() =>
+        issue.setParent('u', parent({ id: 'y', ancestorIds: ['e-1'] }), 1),
+      ).toThrow(/at most 2 levels/);
+    });
+  });
+
+  describe('checklist', () => {
+    it('adds, ticks, rewords and removes steps, and survives a replay', () => {
+      const issue = newIssue();
+      issue.addChecklistItem('u-dev', 'c-1', '  Write the migration ');
+      issue.addChecklistItem('u-dev', 'c-2', 'Update the docs');
+      issue.addChecklistItem('u-dev', 'c-1', 'Duplicate id is ignored');
+      issue.updateChecklistItem('u-dev', 'c-1', { done: true });
+      issue.updateChecklistItem('u-dev', 'c-1', {
+        done: true,
+        text: 'Write the migration',
+      });
+      issue.updateChecklistItem('u-dev', 'c-2', { text: 'Update the README' });
+      issue.removeChecklistItem('u-dev', 'c-2');
+
+      expect(issue.checklist).toEqual([
+        { id: 'c-1', text: 'Write the migration', done: true },
+      ]);
+      expect(replay(issue, Issue).rebuilt.checklist).toEqual(issue.checklist);
+      // created + 2 added + ticked + reworded + removed
+      expect(issue.getUncommittedEvents()).toHaveLength(6);
+    });
+
+    it('refuses empty steps and unknown items', () => {
+      const issue = newIssue();
+      expect(() => issue.addChecklistItem('u', 'c-1', '   ')).toThrow(
+        ValidationError,
+      );
+      expect(() => issue.removeChecklistItem('u', 'nope')).toThrow(
+        BusinessRuleError,
+      );
+    });
+  });
+
   describe('mentions', () => {
     it('reads who a text mentions, once each', () => {
       const text =