fix(policy): remove blanket /Volumes read grant from system_read_macos

[SequeI]

Linked Issue

Closes #727
Ref #1482

Summary

The system_read_macos group granted read access to the entire /Volumes mount namespace (all external drives, disk images, and network shares), described only as paths "required for executables to function." No executable or system path resolution actually needs this — unlike /System/Volumes, which stays in the list because its required to match APFS firmlink-resolved paths (see docs/cli/internals/seatbelt.mdx).

This mirrors the fix already applied to the macOS runtime baseline in a9263ed, which removed an analogous overbroad /System/Volumes grant for the same reason: it could inadvertently expose user data. The system_read_macos policy group never got the same scrutiny.

Reported in discussion #1482, where an external drives contents were readable by every sandboxed process by default with no way to opt out short of a custom deny + --bypass-protection workaround.

Adds a regression test asserting /Volumes is absent from system_read_macos while /System/Volumes remains present.

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	+26
Lines removed	-1
Total changed	27
Classification	Small (< 50 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,configuration / policy files

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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.}