docs: recommend using package for message types

[talos]

Description

Adds a new "Using OpenRouter Types" section to the README that recommends using this package for applications that need to work with OpenRouter's message types directly. The section includes an example import statement showing the available types and notes that the package is tree-shakeable, so importing only types will not impact bundle size.

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

Note: A changeset is required for your changes to trigger a release. If your PR only contains docs, tests, or CI changes that don't need a release, run pnpm changeset --empty instead.

Review & Testing Checklist for Human

• Verify the listed types (ChatCompletionMessageParam, ChatCompletionUserMessageParam, etc.) are actually exported from the package's public API
• Confirm the tree-shakeability claim is accurate (package.json has "sideEffects": false)

Notes

Requested by John Krauss (@talos)

Link to Devin run: https://app.devin.ai/sessions/eca4333dd49e4d889eeb75ecad5cff80

[devin-ai-integration]

Original prompt from John

@Devin update the readme for the typescript sdk to recommend using the package for applications that would like to use our message types. include a note that the package is tree-shakeable and therefore should not impact bundle size provided only the types are used.
Thread URL: https://openrouter.slack.com/archives/C07UF9XLTFF/p1765927926655109

[devin-ai-integration]

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