Adds a "Using Sandcastle for agent dispatch" section between the gate
table and Troubleshooting. Covers when to use / not use sandcastle,
prerequisites (Docker + agent API key + .sandcastle/ config), the
dispatch flow, a worked end-to-end example (plan → execute → review →
manual state mutation), troubleshooting (env vars, Docker, timeouts,
rejection modes), and a cost-aware planning-only variant.
