Add Daytona devcontainer

[bong000]

Summary

• Add a secret-free Dev Container config for running Omni Engineer in Daytona or VS Code Dev Containers.
• Install Python dependencies from requirements.txt during workspace creation.
• Pass OPENROUTER_API_KEY from the developer's local environment instead of committing secrets.

Validation

• Get-Content .devcontainer/devcontainer.json | ConvertFrom-Json
• python -m py_compile main.py
• git diff --check