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/graphql/issues.graphql

@@ -66,6 +66,18 @@ mutation UnlinkIssues($input: LinkIssuesInput!) {
   }
 }
 
+mutation WatchIssue($issueId: ID!) {
+  watchIssue(issueId: $issueId) {
+    ...CommandResultFields
+  }
+}
+
+mutation UnwatchIssue($issueId: ID!) {
+  unwatchIssue(issueId: $issueId) {
+    ...CommandResultFields
+  }
+}
+
 mutation SetIssueRelease($issueId: ID!, $releaseId: ID) {
   setIssueRelease(issueId: $issueId, releaseId: $releaseId) {
     ...CommandResultFields