fix(ci): fix case-sensitive username in unassign step

The `gh issue edit --remove-assignee` is case-sensitive. Using `boshen`
(lowercase) silently succeeded without actually removing `Boshen`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Boshen

.github/workflows/claude.yml

@@ -47,4 +47,4 @@ jobs:
         if: always()
         env:
           GH_TOKEN: ${{ github.token }}
-        run: gh issue edit ${{ github.event.issue.number }} --remove-assignee boshen
+        run: gh issue edit ${{ github.event.issue.number }} --remove-assignee Boshen