feat: environment hook system (pre/post-activate)

[nmhjklnm]

Summary

Allow users to run custom scripts on environment lifecycle events.

Hook points

~/.cac/envs/<name>/hooks/
├── pre-activate.sh       # before activation
├── post-activate.sh      # after activation
├── pre-deactivate.sh     # before switching away
└── pre-claude.sh         # before claude starts (inside wrapper)

Use cases

• Start/stop VPN on environment switch
• Switch DNS settings
• Log environment usage
• Run custom proxy health checks
• Set additional environment variables

Implementation

• Wrapper (templates.sh) checks for hooks/pre-claude.sh and sources it
• _env_cmd_activate checks for pre-activate.sh / post-activate.sh
• Hooks are optional — no hooks directory = no action