[ARI-65] Pipeline REST API + API mode for agent-to-agent communication

[bjridicodes]

Linear: ARI-65

Type: 🚀 Feature | Milestone: M6

Add the full pipeline endpoint and implement API mode — where agents call each other via HTTP instead of in-process LangGraph calls.

Pipeline endpoint

POST /api/v1/pipeline/run

// Request
{ "incident_number": "INC0010001" }

// Response — full PipelineState as JSON
{
  "status": "success",
  "incident_number": "INC0010001",
  "duration_ms": 8432,
  "data": {
    "incident_metadata": { ... },
    "log_result": { ... },
    "classification": { ... },
    "notification_sent": true,
    "error": null
  }
}

API mode

When ARIA_MODE=api is set, agents call each other via HTTP:

• Agent 2 calls POST /api/v1/agent1/run instead of instantiating Agent 1 directly
• Each agent's base URL is configurable: ARIA_AGENT1_URL, ARIA_AGENT2_URL, etc.
• Allows agents to run as separate services (microservice deployment)

Modes

Mode	How agents communicate	Use case
workflow (default)	In-process LangGraph	Single-server deployment
api	HTTP calls between agents	Distributed / microservice deployment

Env var

ARIA_MODE=workflow  # or api