feat(misc): add model routing notebook for cost-optimized task dispatch

Teaches ModelRouter class: Haiku-based difficulty classifier (1-5 scale),
routing to Haiku/Sonnet/Opus, retry-with-upgrade on thin outputs.
Benchmarks 8 tasks, shows 65% cost saving vs always-Opus at scale,
and covers failure modes including domain overrides.

Closes #714

Author: RudrenduPaul