Clarify Vercel and local setup paths

[fmhall]

Summary

• recommend the guided Deploy with Vercel flow to setup agents and explain which services and credentials it provisions automatically
• replace the ambiguous local snippet with explicit Node, pnpm, Docker, Kernel, AI Gateway, and .env.local requirements
• link manual users to Kernel API key creation and document vercel integration add kernel --plan FREE for an existing linked Vercel project
• load .env.local before the development supervisor starts and reject a missing Kernel key before Docker or migrations
• cover both the early failure and the existing present-key Compose lifecycle

Validation

• pnpm test:app tests/local-development.test.ts — 7 tests passed
• KERNEL_API_KEY= pnpm dev — exited before Docker with the expected manual/Vercel guidance
• pnpm check — 60 test files and 254 tests passed, plus lint, types, formatting, Knip, and boundaries
• KERNEL_API_KEY=build-validation-placeholder pnpm build — passed without making an external Kernel request

Review notes

The Vercel path remains the recommended deployment: its Kernel Marketplace integration, Neon resource, private Blob store, Linq connector, and AI Gateway access are provisioned through the guided flow. Fully manual setup links to kernel.sh; an existing linked Vercel project can provision and connect the free Kernel Developer plan through the current Marketplace CLI. The explicit credentials in .env.example and the preflight apply to manual local development.