feat: add smart routing pattern detector

[saurabhhhcodes]

Closes #18

Summary

• add a bounded regex-based detectByPatterns() smart-routing detector
• return task type, confidence, selected model, reason, matched regexes/keywords, timeout status, and elapsed time
• cover code generation/review, math, document analysis, Chinese, data extraction, custom patterns, bounded long input, and empty prompts

Validation

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

[saurabhhhcodes]

Closing this as a duplicate of the existing pattern-detector PR #44 for issue #18. I created this from a fresh branch after noticing an old remote branch conflict, but #44 is already open and mergeable, so keeping one review thread is cleaner for maintainers.