Add output_key field to readiness probes for command nodes

[peter-adam-dy]

Problem

When a command node has a probes.readiness with type "port" or "http", the orchestrator needs to know which output key holds the port/URL to check. Currently it guesses by searching for hardcoded key names (PORT, DB_PORT, SERVICE_PORT for port; URL, DATABASE_URL, SERVICE_URL for http).

If the user's output uses a different key name (e.g., REDIS_PORT, API_URL), the probe silently skips with a warning.

Proposed fix

Add an optional output_key field to HealthCheck:

"probes": {
  "readiness": {
    "type": "port",
    "output_key": "REDIS_PORT",
    "timeout_seconds": 30
  }
}

When output_key is set, use that specific output. When omitted, fall back to the current heuristic search.

Context

Identified during review of #111 (liveness probes).