[P17b] Implement BusClient — server-side HTTP publisher

[four-bytes-robby]

Purpose

Server-side client for plugin bus. Plugins import BusClient, read port file, publish messages via HTTP POST.

Scope

BusClient class

class BusClient {
  static async connect(): Promise<BusClient>
  async publish(channel: string, payload: unknown): Promise<void>
  async healthCheck(): Promise<boolean>
  get port(): number
}

Discovery

• Read ~/.cache/opencode/plugin-bus/port.json → extract port
• Retry up to 3s if file doesn't exist yet (bus starting up)
• Cache port in memory

Publish

• HTTP POST to http://127.0.0.1:{port}/publish
• JSON body: { channel, payload }
• Timeout: 5s
• Error handling: throw on non-200, log warning on timeout

Files

• src/bus-client.ts
• src/discovery.ts (shared with BusTui)

Acceptance

• Unit tests with mock HTTP server
• Integration test: real Go bus, publish message, verify received