feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

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

libs/web/tracker/infrastructure/src/lib/adapters/graphql-issues.gateway.ts

@@ -25,6 +25,8 @@ import {
   SearchIssuesDocument,
   SetIssueReleaseDocument,
   UnlinkIssuesDocument,
+  UnwatchIssueDocument,
+  WatchIssueDocument,
   UpdateIssueDocument,
 } from '../graphql/generated';
 import { GraphqlOperations } from './graphql-operations';
@@ -119,6 +121,16 @@ export class GraphqlIssuesGateway extends IssuesGateway {
       .pipe(map((d) => toCommandResult(d.unlinkIssues)));
   }
 
+  watch(issueId: string, watching: boolean): Observable<CommandResult> {
+    return watching
+      ? this.gql
+          .mutate(WatchIssueDocument, { issueId })
+          .pipe(map((d) => toCommandResult(d.watchIssue)))
+      : this.gql
+          .mutate(UnwatchIssueDocument, { issueId })
+          .pipe(map((d) => toCommandResult(d.unwatchIssue)));
+  }
+
   setRelease(
     issueId: string,
     releaseId: string | null,