feat: validate labels exist in repo before syncing

Adds a pre-flight check in execute_sync that fetches all label names
from the target repository and raises a ValueError listing any labels
referenced in TASKS.md that don't exist yet. This prevents the
perpetual-update loop where the tool detects a label diff, silently
fails to resolve the label IDs, and reports the same 5 updates on
every run.

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

Author: jack-champagne

tasksmd_sync/github_projects.py

@@ -600,6 +600,24 @@ def set_issue_labels(self, issue_id: str, label_ids: list[str]) -> None:
         """
         self._graphql(mutation, {"issueId": issue_id, "labelIds": label_ids})
 
+    def list_label_names(self, repo_owner: str, repo_name: str) -> set[str]:
+        """Return the set of label names (lowercased) defined in a repository."""
+        query = """
+        query($owner: String!, $name: String!) {
+          repository(owner: $owner, name: $name) {
+            labels(first: 100) {
+              nodes { name }
+            }
+          }
+        }
+        """
+        data = self._graphql(query, {"owner": repo_owner, "name": repo_name})
+        return {
+            node["name"].lower()
+            for node in data["repository"]["labels"]["nodes"]
+            if node.get("name")
+        }
+
     def resolve_label_ids(
         self, repo_owner: str, repo_name: str, label_names: list[str]
     ) -> list[str]:

tasksmd_sync/sync.py

@@ -159,6 +159,9 @@ def execute_sync(
     """
     result = SyncResult()
 
+    if repo_owner and repo_name:
+        _validate_labels(client, task_file, repo_owner, repo_name)
+
     logger.info("Fetching current board state...")
     board_items = client.list_items()
     logger.info("Found %d items on the board", len(board_items))
@@ -382,6 +385,30 @@ def _title_fallback_match(
     return None
 
 
+def _validate_labels(
+    client: GitHubProjectClient,
+    task_file: TaskFile,
+    repo_owner: str,
+    repo_name: str,
+) -> None:
+    """Raise ValueError if any labels in TASKS.md don't exist in the repository.
+
+    This prevents silent label-sync failures caused by referencing label names
+    that haven't been created yet.
+    """
+    wanted = {label for task in task_file.tasks for label in task.labels}
+    if not wanted:
+        return
+    existing = client.list_label_names(repo_owner, repo_name)
+    missing = sorted(label for label in wanted if label.lower() not in existing)
+    if missing:
+        raise ValueError(
+            f"TASKS.md references label(s) that don't exist in "
+            f"{repo_owner}/{repo_name}: {', '.join(missing)}. "
+            f"Create them first or remove them from TASKS.md."
+        )
+
+
 def _needs_update(task: Task, board_item: ProjectItem) -> bool:
     """Check if a task's fields differ from the board item."""
     if task.title != board_item.title: