[P17b] Implement BusTui — WebSocket subscriber for TUI plugins

[four-bytes-robby]

Purpose

TUI-side client for plugin bus. TUI components import BusTui, connect WebSocket, subscribe to channels with callbacks.

Scope

BusTui class

class BusTui {
  static async connect(): Promise<BusTui>
  subscribe(pattern: string, callback: (msg: BusMessage) => void): () => void
  async publish(channel: string, payload: unknown): Promise<void>
  close(): void
}

WebSocket connection

• Connect to ws://127.0.0.1:{port}/subscribe?channels={patterns}
• Parse JSON messages: { channel, payload }
• Re-subscribe on reconnect (accumulate patterns)

Auto-reconnect

• On disconnect: retry with exponential backoff (1s, 2s, 4s, max 30s)
• Re-read port file on reconnect (port may have changed)
• Fire onConnect/onDisconnect callbacks

Bidirectional

• publish() sends message back to bus via WebSocket JSON message
• Bus routes to other subscribers

Files

• src/bus-tui.ts
• src/discovery.ts (shared module)

Acceptance

• Unit tests with mock WebSocket server
• Integration test: connect to Go bus, subscribe, publish from server side, verify callback fires