Windows uv venv activation

[NetZissou]

The README installation section

uv venv --python 3.12 && source .venv/bin/activate

Two issues with this

• the && was confusing for some users, so break it down to two commands
• workshop participants with Windows OS were confused because this doesn't work on their machines

Update the Installation section in README.md to cover both shells. For example, using tabbed/OS-split instructions:

# macOS / Linux:
source .venv/bin/activate

# Windows Powershell
.venv\Scripts\Activate.ps1

# Windows Git Bash
source .venv/Scripts/activate

[egrace479]

They should be clarified in separate copy sections (e.g., so Mac users don't also get the Windows install when copying theirs).