Add `install.ps1` — Windows install script

[raphasouthall]

NeuroStack's `install.sh` works on Linux and macOS. Windows users have to install manually via `pip`. A PowerShell install script (`install.ps1`) would make onboarding easier for Windows users.

The script should:

1. Check Python 3.11+ is available
2. Run `pip install neurostack` (or `neurostack[full]` based on a parameter)
3. Print the MCP config snippet they need to add to their AI tool config
4. Optionally check whether Ollama is installed if full mode is requested

Reference: Look at `install.sh` for the overall structure and what checks it performs.

Notes:

• The config file on Windows will be at `%APPDATA%\neurostack\config.toml` — check `src/neurostack/config.py` to see how the path is resolved and whether Windows is already handled
• Do not add Windows to the official requirements list until CI is set up for it — the script can note it's experimental

Acceptance criteria:

• `install.ps1` installs neurostack via pip
• Script checks for Python 3.11+ and prints a clear error if not found
• Script prints the correct MCP config path for Windows
• Script accepts a `-Mode full` parameter to install `neurostack[full]`
• README updated with a Windows install section pointing to the script