fix: consolidate pricing tables to single source of truth

[ntegrals]

Summary

The codebase had two separate pricing tables with conflicting data:

• PRICING_TABLE in agent/types.ts — 10 models, stale pricing, used by calculateStepCost()
• DEFAULT_COST_RATES in metering/types.ts — 24+ models with current pricing, used by UsageMeter

This caused calculateStepCost() to return undefined for newer models (Claude 4, o1, o3-mini, DeepSeek, Mistral, Gemini 2.5), making agent cost reports incomplete.

Fix

• Removed the duplicate PRICING_TABLE constant and PricingTable interface from agent/types.ts
• Re-exported DEFAULT_COST_RATES as PRICING_TABLE for backward compatibility
• Re-exported PricingTable type from metering
• Updated calculateStepCost() to use DEFAULT_COST_RATES directly

Models now covered by cost tracking

Provider	Models
OpenAI	gpt-4o, gpt-4o-mini, gpt-4-turbo, gpt-4.5-preview, o1, o1-mini, o1-preview, o3-mini
Anthropic	claude-3-5-sonnet, claude-3-5-haiku, claude-3-opus, claude-3-haiku, claude-4-sonnet, claude-4-opus
Google	gemini-1.5-pro, gemini-1.5-flash, gemini-2.0-flash, gemini-2.0-pro, gemini-2.5-pro, gemini-2.5-flash
Mistral	mistral-large, mistral-small, codestral
DeepSeek	deepseek-chat, deepseek-reasoner

Backward compatibility

• PRICING_TABLE is still exported (as an alias for DEFAULT_COST_RATES)
• calculateStepCost() signature unchanged
• AgentPricingTable type still exported from index.ts

Test plan

• bun run build — all 3 packages compile clean
• bun run test — all 364 tests pass
• calculateStepCost('claude-4-sonnet', ...) now returns a cost (previously undefined)