docs: enhance direct composition instructions and examples in README

[babblebey]

This pull request updates the documentation to clarify how to handle plugin configuration when using direct composition with the library. It adds a detailed example for wrappers that need to inspect and enforce required plugins before passing them to the core, and clarifies the authoritative source of plugins in direct composition scenarios.

Documentation improvements for direct composition:

• Expanded the explanation in the README.md to clarify that configOptions.buildPlugins: false is useful when wrapper code needs to read and enforce required plugins before passing them explicitly to the core.
• Added a new example to the README.md demonstrating how a wrapper can use resolveConfig(..., { buildPlugins: false }) to read configured plugins, apply policy (such as ensuring required plugins), and then pass the final plugin list to the core.
• Clarified that in the direct-composition path, the explicit plugins argument is always the authoritative source, regardless of whether it is constructed by the caller or a wrapper, and that options.plugins is not used as a fallback once plugins is provided.

[github-actions]

🎉 This PR is included in version 1.0.0-beta.3 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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