Return ML-predicted duration estimates for all open/claimed tasks.

Uses the local GradientBoosting duration predictor. Falls back to the static cold-start table when fewer than 50 completions are available.

Response shape::

{
  "predictor": {
    "trained": true,
    "training_samples": 142,
    "cold_start": false
  },
  "tasks": [
    {
      "task_id": "abc123",
      "title": "Refactor auth module",
      "role": "backend",
      "p50_seconds": 720.0,
      "p90_seconds": 1440.0,
      "confidence": 0.62,
      "is_cold_start": false,
      "eta_p50": "12m 0s",
      "eta_p90": "24m 0s"
    }
  ]
}