Implement label syncing: support label removal from issues

.gitignore

@@ -6,3 +6,4 @@ build/
 .pytest_cache/
 *.egg
 .env
+.venv/

tasksmd_sync/sync.py

@@ -407,7 +407,7 @@ def _needs_update(task: Task, board_item: ProjectItem) -> bool:
                 board_item.assignee,
             )
             return True
-        if task.labels and sorted(task.labels) != sorted(board_item.labels):
+        if sorted(task.labels) != sorted(board_item.labels):
             logger.debug(
                 "  [DIFF] '%s' labels: %r != %r",
                 task.title,
@@ -458,21 +458,25 @@ def _apply_task_fields(
                 logger.warning(
                     "Could not resolve GitHub user '%s' for assignee", task.assignee
                 )
-        if task.labels and sorted(task.labels) != sorted(board_item.labels):
+        if sorted(task.labels) != sorted(board_item.labels):
             # We need repo owner/name to resolve label IDs
             owner = board_item.repo_owner or client.org
             name = board_item.repo_name
             if name:
-                label_ids = client.resolve_label_ids(owner, name, task.labels)
-                if label_ids:
-                    client.set_issue_labels(board_item.content_id, label_ids)
+                if task.labels:
+                    label_ids = client.resolve_label_ids(owner, name, task.labels)
+                    if label_ids:
+                        client.set_issue_labels(board_item.content_id, label_ids)
+                    else:
+                        logger.debug(
+                            "Could not resolve label IDs for %r in %s/%s",
+                            task.labels,
+                            owner,
+                            name,
+                        )
                 else:
-                    logger.debug(
-                        "Could not resolve label IDs for %r in %s/%s",
-                        task.labels,
-                        owner,
-                        name,
-                    )
+                    # Task has no labels — clear all labels from the issue
+                    client.set_issue_labels(board_item.content_id, [])
             else:
                 logger.debug(
                     "Cannot resolve labels for '%s': no repository information found",

tests/test_execute_sync.py

@@ -595,6 +595,32 @@ def test_unresolvable_labels_does_not_crash(self):
         assert result.updated == 1
         client.set_issue_labels.assert_not_called()
 
+    def test_remove_all_labels_from_issue(self):
+        """Labels should be cleared when task has no labels but issue does."""
+        board = [
+            _make_board_item(
+                "PVTI_1",
+                title="Task",
+                status="Todo",
+                content_type="Issue",
+                content_id="I_1",
+                labels=["bug", "docs"],
+                repo_name="tasksmd-sync",
+            ),
+        ]
+        client = _mock_client(board)
+        tf = TaskFile(
+            tasks=[
+                _make_task("Task", board_id="PVTI_1", labels=[]),
+            ]
+        )
+
+        result = execute_sync(client, tf)
+
+        assert result.updated == 1
+        client.resolve_label_ids.assert_not_called()
+        client.set_issue_labels.assert_called_once_with("I_1", [])
+
     def test_update_error_recorded(self):
         """Errors during update should be captured in result.errors."""
         board = [
@@ -785,11 +811,11 @@ def test_no_diff_when_task_has_no_assignee(self):
         board = _make_board_item("X", title="T", content_type="Issue", assignee="bob")
         assert _needs_update(task, board) is False
 
-    def test_no_diff_when_task_has_no_labels(self):
-        """If task has no labels, board's labels shouldn't cause a diff."""
+    def test_labels_removed_when_task_has_no_labels(self):
+        """If task has no labels but board has labels, it SHOULD cause a diff."""
         task = _make_task("T")
         board = _make_board_item("X", title="T", content_type="Issue", labels=["bug"])
-        assert _needs_update(task, board) is False
+        assert _needs_update(task, board) is True
 
     def test_label_order_irrelevant(self):
         """Labels should be compared as sets (order doesn't matter)."""
@@ -953,6 +979,25 @@ def test_issue_without_content_id_skips_assignee_labels(self):
         client.resolve_user_id.assert_not_called()
         client.set_issue_labels.assert_not_called()
 
+    def test_issue_clears_labels_when_task_has_none(self):
+        """Real Issue should have labels cleared when task has no labels."""
+        client = _mock_client()
+        fields = _stub_fields()
+        task = _make_task("T", labels=[])
+        bi = _make_board_item(
+            "PVTI_1",
+            title="T",
+            content_type="Issue",
+            content_id="I_1",
+            labels=["bug", "docs"],
+            repo_name="tasksmd-sync",
+        )
+
+        _apply_task_fields(client, "PVTI_1", task, fields, board_item=bi)
+
+        client.resolve_label_ids.assert_not_called()
+        client.set_issue_labels.assert_called_once_with("I_1", [])
+
 
 # ===================================================================
 # _parse_item_node

tests/test_sync.py

@@ -403,3 +403,41 @@ def test_issue_unchanged_when_labels_match():
     plan = build_sync_plan(tf, board)
     assert len(plan.unchanged) == 1
     assert len(plan.update) == 0
+
+
+def test_label_removal_triggers_update_for_issue():
+    """Removing all labels from a task should trigger an update on a real Issue."""
+    tf = TaskFile(
+        tasks=[
+            _make_task("Task", board_id="PVTI_1", labels=[]),
+        ]
+    )
+    board = [
+        _make_board_item(
+            "PVTI_1",
+            title="Task",
+            content_type="Issue",
+            labels=["bug", "docs"],
+        ),
+    ]
+    plan = build_sync_plan(tf, board)
+    assert len(plan.update) == 1
+
+
+def test_partial_label_removal_triggers_update_for_issue():
+    """Removing some labels from a task should trigger an update on a real Issue."""
+    tf = TaskFile(
+        tasks=[
+            _make_task("Task", board_id="PVTI_1", labels=["bug"]),
+        ]
+    )
+    board = [
+        _make_board_item(
+            "PVTI_1",
+            title="Task",
+            content_type="Issue",
+            labels=["bug", "docs"],
+        ),
+    ]
+    plan = build_sync_plan(tf, board)
+    assert len(plan.update) == 1