Add tests for env pass-through feature:
- Test env command exists and lists files
- Test env set creates env file
- Test env show masks sensitive values (GITEA_TOKEN)
- Test set -a exports variables to child processes
- Test pm-agent.env takes precedence over default.env
- Test cmd_init creates env template files
These tests ensure the env pass-through mechanism works correctly
and that variables are properly exported to subagents.
454a019721