Plugin loader throws TypeError or executes arbitrary named utility exports in legacy plugins

Description

In packages/opencode/src/plugin/index.ts, getLegacyPlugins iterates over all exported values of a plugin module (Object.values(mod)):

for (const entry of Object.values(mod)) {
  if (seen.has(entry)) continue
  seen.add(entry)
  const plugin = getServerPlugin(entry)
  if (!plugin) throw new TypeError("Plugin export is not a function")
  result.push(plugin)
}

This causes two issues when authoring plugins:

1. If a plugin file contains export default async function(ctx) { ... } alongside non-function exports (e.g. export const config = { ... } or constants), getServerPlugin returns undefined for non-functions and it throws TypeError: Plugin export is not a function, failing plugin load.
2. If a plugin exports helper functions (e.g. for unit testing), OpenCode treats every exported function as a plugin initializer and invokes it with (input, load.options), causing unexpected calls or exceptions when the helper expects different arguments.

OpenCode version

1.18.31

Steps to reproduce

1. Create a plugin file exporting both a default plugin function and a constant:

export const MY_CONFIG = { foo: "bar" }
export default async function(ctx) {
  return {}
}

2. Run opencode.
3. Loading fails with TypeError: Plugin export is not a function.

Operating System

Linux

Start in packages/opencode/src/plugin/index.ts at getLegacyPlugins and inspect getServerPlugin and the legacy plugin loading path. Reproduce the examples from the issue, then verify that non-plugin exports do not cause loading errors and helper exports are not invoked as plugin initializers.