uv workspaces for monorepos

Use uv workspaces for multi-package Python projects:

```toml
# Root pyproject.toml
[tool.uv.workspace]
members = ["packages/*"]

[tool.uv.sources]
mylib = { workspace = true }
```

Structure:
```
monorepo/
├── pyproject.toml # Workspace root
├── uv.lock # Single lockfile for all packages
└── packages/
├── mylib/
│ ├── pyproject.toml
│ └── src/mylib/
└── myapp/
├── pyproject.toml # depends on mylib
└── src/myapp/
```

Benefits:
- Single lockfile across all packages
- Local package editable installs
- Atomic dependency updates