feat: cost estimation per task from token counts

[jpleva91]

Problem

Token counts are captured per task but not converted to cost. Need cost estimation for budget tracking.

Acceptance Criteria

• Add cost_usd column to sentinel report (input_tokens * price + output_tokens * price)
• Use DeepSeek pricing: $0.27/M input, $1.10/M output
• Add total run cost to report header

Hints

• Token data at task_result.agent_result.n_input_tokens / n_output_tokens
• Some tasks already have cost_usd field

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