Implement CostTracker with daily budget limits

[bug-ops]

Epic: #366

Description

Track token usage and estimated cost per model with configurable daily budget cap.

Design

[cost]
enabled = true
max_daily_cents = 500

Tasks

• CostTracker struct with record_usage(model, prompt_tokens, completion_tokens)
• Per-model pricing table (configurable, defaults for Claude/OpenAI/Ollama)
• check_budget() -> Result<(), BudgetExhausted>
• Agent loop calls check_budget() before each LLM request
• Daily reset at midnight UTC
• TUI metrics panel shows current spend / budget
• Unit tests with mock pricing