feat: generate SRE orchestrator config as default init output #266

Replace the bare single-agent assistant config from aura-cli init with a full SRE orchestrator config.

Changes

• Coordinator agent with scratchpad and system prompt for SRE task routing
• Three active workers: incident-responder, metrics-analyst, log-analyst
• Two commented-out workers: ops-engineer, runbook-engineer
• MCP server placeholders with commented examples
• Standalone CLI .env discovery from config file directory
• Orchestration routing: explicit "none configured" label for tool-less workers, respond_directly exception when workers lack tools

Motivation

The simple assistant config doesn't showcase AURA's orchestration capabilities. New users should see the full multi-worker SRE experience out of the box, with guidance to add their own MCP servers.