fix(profile): empty allow_vars no longer strips all env vars

[SequeI]

Linked Issue

Ref #1203

Summary

When a profile contained an environment block without explicit allow_vars (e.g. only deny_vars or set_vars), allowed_env_vars resolved to Some([]) instead of None. Because Some([]) activates the allow-list filter with nothing in it, every inherited env var was silently stripped — including PATH and HOME.

Fix: change .map to .and_then and return None when allow_vars is empty, consistent with the existing deny_vars handling.

Test Plan

Checklist

• An issue exists and is linked above
• All commits are signed-off, using DCO
• All new code follows the project's coding standards (CLAUDE.md) and is covered by tests
• Public-facing changes are paired with documentation updates
• Release note has been added to CHANGELOG.md if needed

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+124
Lines removed	-24
Total changed	148
Classification	Medium (50–300 lines)

Affected crates

• crates/nono-cli — CLI changes. Verify argument parsing, flag documentation, and UX behaviour across supported platforms.

Blast radius — Moderate

This PR touches: source code,documentation

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[gemini-code-assist]

Code Review

This pull request updates the environment variable filtering logic in profile_runtime.rs so that an empty allow_vars list returns None instead of activating an empty allow filter (which would block all environment variables). The feedback points out that the new tests duplicate production logic in a helper function, which can lead to test drift. It is recommended to refactor the tests to use the actual prepare_profile entry point with temporary files on disk to ensure the real production code path is validated.

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