Do not stale issues with enhancement tag (#59)

<!-- Korbit AI PR Description Start -->
## Description by Korbit AI

### What change is being made?

Update the GitHub Actions workflow configuration to exclude issues with
the "enhancement" label from being marked as stale.

### Why are these changes being made?

Issues labeled as "enhancement" are typically longer-term improvements
which might not receive frequent updates. Exempting them from being
automatically flagged as stale ensures that valuable enhancement ideas
are retained for review without premature closure.

> Is this description stale? Ask me to generate a new description by
commenting `/korbit-generate-pr-description`
<!-- Korbit AI PR Description End -->

.github/workflows/issue_stale.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/stale@v9
         with:
           operations-per-run: 250
-          exempt-issue-labels: "in progress"
+          exempt-issue-labels: "in progress,enhancement"
           days-before-pr-stale: -1
           days-before-pr-close: -1
           days-before-issue-stale: 30