Refactor shell integration into dedicated package

[jestenough]

Summary

• Add a dedicated pcd_cli.integrations package.
• Move shell integration implementation from:
  • pcd_cli.shell_integration
  • to pcd_cli.integrations.shell
• Keep Click commands in pcd_cli.cli.shell.
• Update all internal imports.
• Rename the related integration test module to match the new structure.

Motivation

This separates CLI command handling from integration-specific implementation and provides a clear location for future integrations such as tmux, editors, and other external tools.

Package structure

pcd_cli/
├── cli/
│   └── shell.py
└── integrations/
    ├── __init__.py
    └── shell.py