feat(v1.2.0): retry API, dernier commit, tri, coloration, export JSON

- client.py: _get_with_retry (max 2 retries, backoff lineaire), get_latest_commit
- collector.py: champ last_commit_date dans RepoData
- display.py: colonne "Dernier commit", _sort_repos (name/issues/release/activity),
  _colorize_milestone_due (rouge/jaune/vert selon echeance)
- cli.py: options --sort/-s et --format/-f (table/json)
- exporter.py: nouveau module, repos_to_dicts + export_json
- 88 tests (35 nouveaux), ruff clean

fixes #8, fixes #7, fixes #10, fixes #9, fixes #6

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

tests/test_collector.py

@@ -130,6 +130,54 @@ class TestCollectAll:
         assert result[0].is_mirror is True
 
 
+class TestCollectAllLastCommit:
+    """Test last_commit_date field in RepoData."""
+
+    def test_repo_data_has_last_commit_date(self):
+        """RepoData includes last_commit_date field."""
+        repo = RepoData(
+            name="test",
+            full_name="admin/test",
+            description="",
+            open_issues=0,
+            is_fork=False,
+            is_archived=False,
+            is_mirror=False,
+            latest_release=None,
+            milestones=[],
+            last_commit_date="2026-03-10T14:30:00Z",
+        )
+        assert repo.last_commit_date == "2026-03-10T14:30:00Z"
+
+    def test_collect_all_calls_get_latest_commit(self):
+        """collect_all calls get_latest_commit and fills last_commit_date."""
+        client = MagicMock()
+        client.get_repos.return_value = [_make_repo()]
+        client.get_latest_release.return_value = None
+        client.get_milestones.return_value = []
+        client.get_latest_commit.return_value = {
+            "sha": "abc123",
+            "created": "2026-03-10T14:30:00Z",
+        }
+
+        result = collect_all(client)
+
+        client.get_latest_commit.assert_called_once_with("admin", "my-repo")
+        assert result[0].last_commit_date == "2026-03-10T14:30:00Z"
+
+    def test_collect_all_no_commits(self):
+        """Repo without commits gets last_commit_date=None."""
+        client = MagicMock()
+        client.get_repos.return_value = [_make_repo()]
+        client.get_latest_release.return_value = None
+        client.get_milestones.return_value = []
+        client.get_latest_commit.return_value = None
+
+        result = collect_all(client)
+
+        assert result[0].last_commit_date is None
+
+
 class TestCollectAllFiltering:
     """Test collect_all filtering (include/exclude)."""