fix(audit): sort milestones, sort JSON export, extract test helper

- Fix milestone section using unsorted repos list
- Apply --sort to --format json output
- Rename _sort_repos to sort_repos (now used by cli.py)
- Extract shared make_repo helper to tests/helpers.py
- Move exporter import to module level in cli.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/helpers.py

@@ -0,0 +1,30 @@
+"""Shared test fixtures and helpers."""
+
+from gitea_dashboard.collector import RepoData
+
+
+def make_repo(
+    name="my-repo",
+    full_name="admin/my-repo",
+    description="A repo",
+    open_issues=3,
+    is_fork=False,
+    is_archived=False,
+    is_mirror=False,
+    latest_release=None,
+    milestones=None,
+    last_commit_date=None,
+):
+    """Build a RepoData for testing."""
+    return RepoData(
+        name=name,
+        full_name=full_name,
+        description=description,
+        open_issues=open_issues,
+        is_fork=is_fork,
+        is_archived=is_archived,
+        is_mirror=is_mirror,
+        latest_release=latest_release,
+        milestones=milestones if milestones is not None else [],
+        last_commit_date=last_commit_date,
+    )