feat: add smart routing pattern detector

[saurabhhhcodes]

Summary

• adds detectByPatterns() for regex/keyword smart-routing task detection
• includes built-in task patterns for code, review, math, reasoning, documents, docs, writing, translation, extraction, factual QA, chat, and Chinese prompts
• returns confidence, matched patterns/keywords, model recommendation, timeout metadata, and unknown fallback results
• exports the detector and TypeScript types from the package entrypoint
• adds focused detector regression tests, including bounded long-input scanning

Validation

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

Closes #18