Summary

Adds a runtime_ms column to chat_messages that records the wall-clock duration (in milliseconds) of each LLM step. This covers LLM streaming, tool execution, and retries — the full time the agent is "alive" for a step.

This is the foundation for billing by agent alive time. The column follows the same pattern as total_cost_micros: stored per assistant message, aggregatable with SUM() over time periods by user.

Changes

• Migration: adds nullable runtime_ms bigint to chat_messages.
• chatloop: adds Runtime time.Duration field to PersistedStep, measures time.Since(stepStart) at the beginning of each step (covering stream + tool execution + retries).
• chatd: passes step.Runtime.Milliseconds() to the assistant message InsertChatMessage call; all other message types (system, user, tool) get NULL.
• Tests: adds runtime > 0 assertion in chatloop tests.

Billing query pattern

Once ready, aggregation mirrors the existing cost queries:

SELECT COALESCE(SUM(cm.runtime_ms), 0)::bigint AS total_runtime_ms
FROM chat_messages cm
JOIN chats c ON c.id = cm.chat_id
WHERE c.owner_id = @user_id
  AND cm.created_at >= @start_time
  AND cm.created_at < @end_time
  AND cm.runtime_ms IS NOT NULL;