Valibot adapter: webmcp-tools/valibot

Valibot schemas already validate at runtime via Standard Schema — only JSON Schema **descriptor generation** is missing.

## Task
Add a `webmcp-tools/valibot` subpath export (mirror `src/zod.ts`):
- Use [`@valibot/to-json-schema`](https://github.com/fabian-hiller/valibot/tree/main/packages/to-json-schema)
- Call `registerSchemaConverter("valibot", fn)` as a side effect
- Optional peer dep + tsup entry + exports map entry (copy the `./zod` pattern in `packages/core/package.json`, including the `.d.cts` require condition)
- Tests colocated: descriptor derivation + end-to-end `tool()` registration with a valibot schema

## Pointers
- `packages/core/src/zod.ts` is the 30-line template
- `packages/core/src/schema.ts` → `resolveInputSchema` shows how converters are looked up by `~standard.vendor`