Add dashboard agent details

[cristian-oai]

Context

The dashboard listed running sessions but did not expose enough live detail to understand what an agent was doing.

TL;DR

Add clickable running sessions with agent stage, checklist, and recent Codex activity.

Summary

• Keep a bounded recent Codex update history for active sessions.
• Add execution stage and checklist payloads to the dashboard/API presenter.
• Make running session rows selectable and render an agent detail panel.
• Show current stage, workspace/session metadata, checklist status, and recent events.
• Add responsive styling and tests for the new dashboard detail view.
• Document the running-session detail panel in the Elixir README.

Alternatives

• Continue linking only to JSON details, but that keeps live inspection hard to scan.
• Add a separate page per session, but inline details keep the operations view compact.
• Wait for structured Codex stage events, but the current inferred stage is already useful.

Test Plan

• mise exec -- make all
• mise exec -- mix test test/symphony_elixir/extensions_test.exs test/symphony_elixir/orchestrator_status_test.exs
• mise exec -- mix specs.check