simplify venv activation

[zkurtz]

Feature request: source .venv/bin/activate takes too long to remember and type. Can this be as easy as uv activate or uv start for the default venv name?

[zanieb]

Please see

• [Feature Request] Add uv venv --activate to activate virtual environment after creation #8106 (comment)
• Feature suggestion: uv venv -a/--activate #8118
• uv sync --activate #8715
• Add a command to activate the virtual environment, e.g., uv shell #1910