test: isolate integration suite XDG state

[SequeI]

Linked Issue

Closes #1732

Summary

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
• If this PR introduces a major feature, capability, or security-relevant change, a corresponding NEP has been opened or accepted and is linked

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+131
Lines removed	-100
Total changed	231
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,CI / build tooling

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[nogent-nolabs-ai]

nogent code review

No blocking issues; 0 findings.

Findings: none flagged in scope.

_{Automated code + security review. CI already covers clippy, rustfmt, tests, cargo-audit and commit-lint.}