feat(web): choose agent and model in the New run dialog #75

Parent

#62

What to build

The UI's New run dialog can only start a run on the config's default agent and that agent's
model. #71 added the run level of the agent choice (ADR 0013 §2): POST /api/runs accepts
agent: { agent?, model? }, and factory start has --agent / --model. The dialog sends
neither, not even from its raw-JSON mode, which edits only the workflow input. In #67's live leg
every opencode run had to be started from the CLI or with curl, and could only be watched and
cancelled from the UI.

• Add an agent select (claude / opencode, defaulting to the config default) and an optional
  model text field to the New run dialog. Send them as the request's agent.
• An empty model means "the agent's configured model" (agent.models[agent]). Factory keeps no
  model catalogue (ADR 0013 §2), so the field is free text.
• Run again should reuse the original run's choice. That needs the run summary to carry the
  run-level agent (today only AgentStepStarted records it).

Acceptance

• From the UI, a run can start on either agent, with or without a model, and its steps'
  AgentStepStarted show the chosen agent and model.
• A playwright test covers the dialog sending agent.