docs: add local web + control plane setup

[sidpalas]

Summary

• Adds a Path C setup flow for running the web app and control plane locally while using real GitHub App credentials and real Modal sandboxes.
• Adds local-dev helpers to generate package env files, sync Modal secrets from .env.local, run local D1 migrations, and start the local web/control-plane services.
• Documents the ngrok/tunnel requirement so remote Modal sandboxes can connect back to the local control plane WebSocket.

Validation

• Verified the real-services local stack end-to-end with local web, local Wrangler control plane, real GitHub, and remote Modal sandbox callback through ngrok.
• Ran npm run dev:env missing-file behavior before .env.local existed.
• Ran npm run dev:modal-secrets successfully to create/update Modal secrets from .env.local.

Future Work

• Reduce GitHub setup requirements by using Vercel emulate to provide local GitHub OAuth/App APIs for development.
• Reduce sandbox setup requirements by adding a local host-based sandbox provider and/or Docker sandbox provider for development.

[sidpalas]

created pr against upstream ColeMurray#573