feat: add smart routing task registry

[saurabhhhcodes]

Summary

• adds TaskRegistry and taskRegistry for Smart Model Selection task configs
• registers the documented built-in task types with models, reasons, keywords, patterns, priorities, and context threshold support
• exports the registry/API from the package entrypoint and adds focused regression tests

Closes #15

Validation

• npm ci
• npm run build
• node tests/task-registry.test.js
• node tests/cross-provider.test.js
• git diff --check