feat: environment variable support per runbook

[HerbHall]

Summary

Allow defining environment variables per runbook that get passed to command execution.

Research Context

• Task (14.9k stars): .env file loading + per-task env vars
• Just (31.7k stars): Per-recipe env vars and .env file support
• mise: Per-task env + tool env configuration

Use Cases

• Compose files that use ${VAR} substitution
• Build args: docker build --build-arg VERSION=$VERSION
• Registry auth: DOCKER_CONFIG pointing to specific credentials
• Multi-environment: Same command, different env vars for staging vs production

Proposed UX

• "Environment Variables" section in RunbookFormDialog
• Key-value pair editor (add/remove rows)
• Variables passed to docker.cli.exec() environment
• Optional: Import from .env file