Add a durable GitHub triage example under examples

[toyamarinyon]

Summary

• add examples/sandbox-github-triage as a SaaS-style GitHub issue/PR triage app
• model triage as durable runCommand() steps for repo sync, context collection, analysis, and report rendering
• add Next.js UI, API routes, Drizzle schema, and migrations for tracked repositories, triage runs, and step history
• document the new example and link it from the root README

Testing

• bun run --cwd packages/sandkit build
• bun run --cwd examples/sandbox-github-triage typecheck
• bun run --cwd examples/sandbox-github-triage build
• bun run --cwd examples/sandbox-github-triage db:migrate