feat(gemset): store and re-use Worktree shell env

[vitallium]

Refactor Gemset to store environment variables in the struct instead
of passing them to each method call. I think there was a change in upstream
how extensions commands inherit the process env. Without re-using Worktree
env the Ruby extension loses ability to use the correct Ruby version and other important variables like PATH or RBENV_DIR.

This also simplifies the API by:

• Adding envs field to the Gemset struct to store environment vars
• Modifying new() to accept optional environment variables
• Removing envs parameters from all public methods
• Merging stored envs with command-specific envs in execute_gem_command
• Updating all method calls and tests to use the new API pattern