fix(plugin): correct exports to point to dist instead of src

The package.json exports were pointing to ./src/*.ts but the published
package only includes the dist/ folder. This caused 'Cannot find module'
errors when custom tools tried to import @opencode-ai/plugin.

Changed exports from:
  ".": "./src/index.ts"
  "./tool": "./src/tool.ts"

To:
  ".": { "types": "./dist/index.d.ts", "import": "./dist/index.js" }
  "./tool": { "types": "./dist/tool.d.ts", "import": "./dist/tool.js" }