chore: add Daytona devcontainer

[Jorel97]

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

What it does:

• Uses the official Python 3.11 devcontainer image.
• Installs uv, runs uv sync, and falls back to pip install -r requirements.txt if needed.
• Passes ANTHROPIC_API_KEY and optional E2B_API_KEY from the local/Daytona environment without committing secrets.
• Copies .env.example to .env only when a local .env does not already exist.
• Forwards port 5000 for the Flask web UI and adds Python/Pylance VS Code extensions.

Validation:

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