Consolidate example install commands

[NathanFlurry]

Summary

• add a shared SANDBOX_AGENT_INSTALL_VERSION constant and generateInstallCommand helper in @sandbox-agent/example-shared
• update the E2B, Daytona, Vercel, Docker, and ComputeSDK examples to use one POSIX install command instead of duplicating curl and install-agent steps
• keep component preinstallation configurable by passing the requested agent components into the shared helper

Testing

• not run: workspace dependencies are not installed, so tsc and the pre-commit biome hook were unavailable locally

[railway-app]

🚅 Environment sandbox-agent-pr-249-70a71eeb-ea2c-41ac-a9d5-acd81967fe5a in sandbox-agent has no services deployed.