Normalize a framework event into Meridian's task_log.

Auth: X-Meridian-Token: <project_token> header. The token grants write-access to a single project's task_log only — call ensure_project_token to mint one (returned by the dashboard's Project settings panel, by GET /projects/{id}/webhook-token).

Body schema (all optional except description+session_name):

{
  "type": "task_completed" | "checkpoint" | "hitl_request" | "session_start",
  "session_name": "langgraph-researcher",
  "human_id": "langgraph",
  "agent_framework": "langgraph",
  "description": "researcher agent fetched 3 sources",
  "status": "done",
  "parent_task_id": null,
  "metadata": {}
}