validate labels before creating issue

fixes #13

- move _validate_labels call before issue creation
- prevents creating orphaned issues when labels are invalid
- fail fast with clear error message listing available labels

src/tangled_mcp/_tangled/_issues.py

···
 
     next_issue_id = max_issue_id + 1
 
+    # validate labels BEFORE creating the issue to prevent orphaned issues
+    if labels:
+        _validate_labels(labels, repo_labels)
+
     # generate timestamp ID for rkey
     tid = int(datetime.now(timezone.utc).timestamp() * 1000000)
     rkey = str(tid)