Fix zod 4 incompatibility in loggerSchema (closes #17)

[bjcoombs]

Summary

Fixes #17: `ai-sdk-v5` tag line (`v0.0.2`) crashes at module-load under zod 4 because `loggerSchema` uses zod 3's `z.function().args().returns()` API, which was removed in zod 4.

Changes

• `src/validation.ts`: Replace `z.function()`-based logger schemas with `z.custom(v => typeof v === 'function')`. Identical behaviour across zod 3 and zod 4, matching the package's declared `zod@^3 || ^4` peer range.

Why `z.custom` rather than `z.function({ input, output })`

`z.function({ input, output })` is zod 4 only and would break zod 3 consumers. `z.custom` is stable across both majors and captures exactly the runtime guarantee this schema needs — "this value is a function". The original `.args(z.string())` signature declaration was never actually validating call-sites anyway; it was documentation in type form, which the TypeScript `Logger` interface already provides.

Testing

• `npm run build` — clean
• `npx tsc --noEmit` — clean
• `npm test` — 269 tests pass
• Verified module loads successfully under both `zod@^3.24.0` and `zod@^4.1.0`:
  ```bash
  npm install --no-save zod@^4.1.0 && node -e "require('./dist/index.cjs').createOpencode({})"

  loaded OK

  ```

Context

I hit this while integrating the provider into claude-task-master#1685. Task Master uses AI SDK v5 which pulls zod 4 through `@ai-sdk/*` packages, so any consumer in that ecosystem is affected.

Test plan

• Build passes
• Typecheck clean
• Existing vitest suite (269 tests) passes
• Verified load under zod 3 and zod 4
• Maintainer review