chore: simayi-approved changes - lint fixes, toolchain improvements, healthz

All changes reviewed and APPROVED in PR #12 (Review ID: 40):
- toolchain_routes: webhook repo/org format compat, content dedup (sha256), closed issue filter
- dispatcher: inform mail crash 误标 done 修复
- ticker: cleanup and improvements
- healthz endpoint
- conftest: integration/e2e deselect markers
- docs: design docs, test-guide updates
- various lint/whitespace fixes across 30 files

src/api/checkpoint_routes.py

@@ -10,7 +10,7 @@ from pydantic import BaseModel
 from typing import Optional
 
 from src.blackboard.operations import Blackboard
-import src.utils as _utils
+from src.utils import get_data_root
 
 router = APIRouter(prefix="/api/projects/{project_id}/tasks/{task_id}/checkpoints", tags=["checkpoints"])
 
@@ -33,7 +33,7 @@ class ResolveCheckpointRequest(BaseModel):
 # ── 工具 ──
 
 def _bb(project_id: str) -> Blackboard:
-    db_path = _utils.get_data_root() / project_id / "blackboard.db"
+    db_path = get_data_root() / project_id / "blackboard.db"
     if not db_path.exists():
         raise HTTPException(status_code=404, detail="Project not found")
     return Blackboard(db_path)