chore: add Daytona devcontainer

[Jorel97]

Adds a minimal Daytona-ready Dev Container for Omni Engineer.

What it does:

• Uses the official Python 3.11 devcontainer image.
• Installs repository dependencies from requirements.txt on workspace creation.
• Passes OPENROUTER_API_KEY from the local/Daytona environment without committing secrets.
• Copies .env.example to .env only when a local .env does not already exist.
• Adds Python/Pylance VS Code extensions for the Daytona workspace.

Validation:

• Parsed .devcontainer/devcontainer.json with Python json.load.
• git diff --check passed.