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/infrastructure/src/lib/adapters/graphql-issues.gateway.ts

@@ -5,6 +5,7 @@ import {
   ChangeStatusInput,
   CommandResult,
   CreateIssueInput,
+  Epic,
   Issue,
   IssuePage,
   IssuesGateway,
@@ -15,18 +16,24 @@ import {
 } from '@task-stream/web/tracker/domain';
 import { map, Observable } from 'rxjs';
 import {
+  AddChecklistItemDocument,
   AssignIssueDocument,
   BoardDocument,
   ChangeIssueStatusDocument,
   CreateIssueDocument,
+  EpicsDocument,
+  IssueChildrenDocument,
   IssueDocument,
   LinkIssuesDocument,
   MyIssuesDocument,
+  RemoveChecklistItemDocument,
   SearchIssuesDocument,
+  SetIssueParentDocument,
   SetIssueReleaseDocument,
   UnlinkIssuesDocument,
   UnwatchIssueDocument,
   WatchIssueDocument,
+  UpdateChecklistItemDocument,
   UpdateIssueDocument,
 } from '../graphql/generated';
 import { GraphqlOperations } from './graphql-operations';
@@ -131,6 +138,61 @@ export class GraphqlIssuesGateway extends IssuesGateway {
           .pipe(map((d) => toCommandResult(d.unwatchIssue)));
   }
 
+  children(projectId: string, issueId: string): Observable<IssueSummary[]> {
+    return this.gql
+      .query(IssueChildrenDocument, {
+        filter: { projectId, parentId: issueId },
+      })
+      .pipe(map((d) => d.issues.items.map(toIssueSummary)));
+  }
+
+  setParent(
+    issueId: string,
+    parentId: string | null,
+  ): Observable<CommandResult> {
+    return this.gql
+      .mutate(SetIssueParentDocument, { issueId, parentId })
+      .pipe(map((d) => toCommandResult(d.setIssueParent)));
+  }
+
+  epics(projectId: string): Observable<Epic[]> {
+    return this.gql.query(EpicsDocument, { projectId }).pipe(
+      map((d) =>
+        d.epics.map((epic) => ({
+          issue: toIssueSummary(epic.issue),
+          progress: { ...epic.progress },
+        })),
+      ),
+    );
+  }
+
+  addChecklistItem(issueId: string, text: string): Observable<CommandResult> {
+    return this.gql
+      .mutate(AddChecklistItemDocument, { input: { issueId, text } })
+      .pipe(map((d) => toCommandResult(d.addChecklistItem)));
+  }
+
+  updateChecklistItem(
+    issueId: string,
+    itemId: string,
+    changes: { text?: string; done?: boolean },
+  ): Observable<CommandResult> {
+    return this.gql
+      .mutate(UpdateChecklistItemDocument, {
+        input: { issueId, itemId, ...changes },
+      })
+      .pipe(map((d) => toCommandResult(d.updateChecklistItem)));
+  }
+
+  removeChecklistItem(
+    issueId: string,
+    itemId: string,
+  ): Observable<CommandResult> {
+    return this.gql
+      .mutate(RemoveChecklistItemDocument, { issueId, itemId })
+      .pipe(map((d) => toCommandResult(d.removeChecklistItem)));
+  }
+
   setRelease(
     issueId: string,
     releaseId: string | null,