fix(rules): anchor system var tmp paths

[be-student]

Fixes #81.

What & why

Anchors the actionable var/tmp rule to Linux /var/tmp and macOS /private/var/tmp. User, project, and Homebrew var/tmp paths now remain unknown instead of being offered for quarantine.

Checklist

• cargo fmt --all and strict clippy pass for diskern-core and diskern-cli
• All 57 core and CLI tests pass
• Commit is one focused safety-boundary change
• Deterministic verdict and quarantine-only safety principles remain intact

[Muawiya-contact]

This one's spot on. Mirroring unix-system-logs is the right shape, and you covered all four cases from the issue plus the macOS path. Rebased and ran it here: fmt, clippy and 57 tests all clean.

Only outstanding thing is the changelog line, same as the other two.

[be-student]

Merged current main and moved the changelog entry to Unreleased. cargo test -p diskern-core passes (61 tests).

[Muawiya-contact]

Changelog's in the right place now, and the rule itself was already right. Ran it here: fmt, clippy and all tests pass.

There's a small conflict on CHANGELOG.md — #92 landed a ### Fixed bullet on the same line yours goes on. Nothing for you to do, both lines just want keeping and I'll sort it on merge. Everything else auto-merges clean, and I checked the combined result builds and passes.

Approved.