feat: Add embedded NATS JetStream coordination bus

Central now starts an embedded NATS JetStream server as its core
coordination bus. This provides:

- wild-services KV bucket: service registrations from Cloud/Works
- wild-presence KV bucket: node liveness with TTL-based keys
- wild-events stream: events from all sources (24h retention)

NATS is started in main.go before the API and shut down on SIGTERM.
Cloud and Works will connect to Central's NATS (port 4222) to register
services and maintain presence.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

go.sum

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