feat: add smart routing strategy

[saurabhhhcodes]

Summary

• add strategy: "smart" support to the model router
• classify prompt/request intent locally from common request body and message shapes
• route high-confidence task types to configurable task-specific models
• fall back to configured smart fallback models when confidence is low or the selected model was already attempted
• validate smart-routing threshold, task model map, and fallback model config

Validation

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

Closes #22

[saurabhhhcodes]

Quick review/scoring follow-up: this PR is clean from my side and has no checks configured in the repo.

Validation already run:

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

Could you please review when you get a chance and add the appropriate GSSoC labels before merge? The linked issue is #22 and the PR currently has no visible scoring labels.