Implement stdlib network and command helpers

[leynos]

Summary

• add MiniJinja stdlib helpers for shell/grep commands that mark templates impure and surface actionable errors
• provide a fetch helper with optional caching and expose the state handle while registering the stdlib
• cover the new behaviour with rstest suites, cucumber features, and updated design documentation and roadmap

Testing

• make lint
• make test
• cargo test --test cucumber

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https://chatgpt.com/codex/tasks/task_e_68e7d0a7807c83228d0dbf3ddd828811