ci: add critical fixes section to release notes (#4361)

Adds a "🚨 Critical Fixes" section to the release notes, directly below
Highlights, for PRs labeled `critical-fix`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/release_notes.json

@@ -6,6 +6,10 @@
             "title": "## 🏆 Highlights",
             "labels": ["highlight"]
         },
+        {
+            "title": "## 🚨 Critical Fixes",
+            "labels": ["critical-fix"]
+        },
         {
             "title": "## 🛠 Breaking Changes",
             "labels": ["breaking-change"]
@@ -38,4 +42,4 @@
             "labels": ["ci"]
         }
     ]
-}
+}