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/test_exporter.py

@@ -2,35 +2,9 @@
 
 import json
 
-from gitea_dashboard.collector import RepoData
 from gitea_dashboard.exporter import export_json, repos_to_dicts
 
-
-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,
-    )
+from tests.helpers import make_repo as _make_repo
 
 
 class TestReposToDicts: