[ARI-63] Agent 3 REST API — /run endpoint returning ClassificationResult as JSON

[bjridicodes]

Linear: ARI-63

Type: 🚀 Feature | Milestone: M4

Expose Agent 3 (Classifier) as a FastAPI endpoint. Accepts incident metadata + log result and returns a classification with confidence scoring.

Endpoints

POST /api/v1/agent3/run

// Request
{
  "incident_number": "INC0010001",
  "incident_metadata": { ... },
  "log_result": { ... }
}

// Response 200
{
  "status": "success",
  "agent": "agent3",
  "incident_number": "INC0010001",
  "duration_ms": 1520,
  "data": {
    "error_class": "resource",
    "error_label": "OOM — YARN container killed",
    "confidence": 0.87,
    "confidence_band": "high",
    "supporting_evidence": ["Container killed by YARN due to memory limit", "GC overhead limit exceeded"],
    "recommended_actions": ["Increase YARN container memory", "Check for memory leak in job X"]
  },
  "error": null
}