feat(events): context and cost per agent step, ACP-aware run detail #65

Parent

#62

What to build

ADR 0013 §5 and the SPA side of the change.

• AgentStepFinished gains optional context: { used, size } and
  cost: { amount, currency }, taken from the last usage signal of the step, including for
  cancelled steps. Update the usage doc comment: "as the agent reports it", which means the
  last message on opencode and the whole turn on Claude.
• Run detail shows context in use and window size from context where present. It falls
  back to agentStepContextTokens for old logs. Cost shows per step and summed per run where
  present.
• Live context: while a step runs, the SPA reads the acp.usage CUSTOM chunks to show
  context filling up.
• Transcript tool titles: ACP tool calls carry a descriptive title (Edit math.ts) in the
  raw event. Show it instead of the generic kind (edit, read, execute) where present.
• Tests for the schema, the fold, and the rendering.

Acceptance

• bun run check and bun run test:e2e green.
• A real Claude run shows context and cost in run detail. Validate in the browser.