[P17b] Add type-safe message contracts and package exports

[four-bytes-robby]

Purpose

Define type-safe message envelope types and configure package.json exports for server and TUI entry points.

Scope

Types (src/types.ts)

interface BusEnvelope<C extends string = string, P = unknown> {
  channel: C;
  payload: P;
  ts: number; // Date.now() when published
}

type BusCallback<C extends string, P> = (msg: BusEnvelope<C, P>) => void;

Package exports

{
  "exports": {
    ".": { "types": "./dist/index.d.ts", "import": "./dist/index.js" },
    "./tui": { "types": "./dist/tui.d.ts", "import": "./dist/tui.js" }
  }
}

Channel namespace types (optional, for consumers)

// Consumers can define their own:
type TbgStatusChannel = "tbg" | "status";
type TbgStatusPayload = { cumulative: number; softLimit: number; hardLimit: number; percentage: number; status: string };

Build

• bun build for server (./index)
• tsc for types (.d.ts)
• bun build for tui (./tui)

Files

• src/types.ts
• src/index.ts (re-export BusClient)
• src/tui.ts (re-export BusTui)
• package.json (exports map)

Acceptance

• bun build succeeds
• Import test: import { BusClient } from "@four-bytes/opencode-plugin-lib" resolves
• Import test: import { BusTui } from "@four-bytes/opencode-plugin-lib/tui" resolves