feat: add smart model routing strategy

[saurabhhhcodes]

Summary

• add strategy: "smart" support to ModelRouter
• add a lightweight TaskClassifier with built-in task rules, custom rule support, model overrides, thresholding, and default-model fallback
• validate smart routing options and block cross-provider selections unless explicitly enabled
• add focused smart-router tests and document the new router options

Closes #23

Validation

• npm run build
• npm run test:smart
• node tests/cross-provider.test.js
• git diff --check

[saurabhhhcodes]

Closing this to avoid duplicating my already-open smart strategy PR #55 for issue #22. Issue #23 should be handled as a small follow-up after #55 lands, so maintainers do not have to review two overlapping ModelRouter implementations at once.