Adopt custom init script copilot SDK API

[anthonykim1]

Adopt: github/copilot-sdk#1911

VS Code could inject part of:

• https://github.com/microsoft/vscode/blob/main/src/vs/workbench/contrib/terminal/common/scripts/shellIntegration.ps1

• https://github.com/microsoft/vscode/blob/main/src/vs/workbench/contrib/terminal/common/scripts/shellIntegration-bash.sh

and other variant for different shell in order to achieve proper Python activation when commands are run with SDK shell tool.

This should set good precedence for other Copilot SDK consumer as well to run their custom scripts (user or embedder defined).