Record a blocker

Records that the task in the path is blocked by `blocker_task_id` — the SAME operation the MCP `link_task_dependency` tool executes. Recording a link that already exists succeeds unchanged (no duplicate row, no second webhook), so retries are safe. A link that would close a loop is REFUSED with 409 `dependency_cycle`: the graph a client reads is guaranteed acyclic. A task cannot block itself (400 `validation_failed`), and a blocker in another organization is indistinguishable from one that does not exist (404 `resource_not_found`). Answers the task's full dependency view, so the caller can see whether it is now blocked without a second call.

curl -X POST "https://example.com/v1/tasks/string/dependencies" \  -H "Content-Type: application/json" \  -d '{    "blocker_task_id": "string"  }'

{  "task_id": "string",  "is_blocked": true,  "blocked_by_count": -9007199254740991,  "blocking_count": -9007199254740991,  "blocked_by": [    {      "task_id": "string",      "title": "string",      "status": "TODO",      "open": true    }  ],  "blocking": [    {      "task_id": "string",      "title": "string",      "status": "TODO",      "open": true    }  ]}