Add per-model cost and cache hit rate breakdown

[li195111]

Summary

Add a new CLI command or report section that shows cost and cache hit rate broken down by model (e.g., Opus, Sonnet, Haiku).

Current Behavior

cta cost-report shows aggregate costs. Model-level breakdown requires manual session inspection.

Expected Behavior

A cta model-breakdown command (or --by-model flag) that shows:

Model	Sessions	Total Cost	Avg Cache Hit	Avg Cost/Session
claude-opus-4-6	120	$2,340	45%	$19.50
claude-sonnet-4-6	5,200	$3,100	72%	$0.60

Implementation Hints

• storage.rs already stores model information per session
• Add a query method that groups by model
• Use existing analyzer.rs aggregation patterns as reference

Acceptance Criteria

• Per-model breakdown displayed with cost, session count, cache hit rate
• Works as both CLI command and MCP tool
• Handles unknown/fallback models gracefully