fix(actions): shorten quarantine names for Windows paths

[Muawiya-contact]

What changed

This fixes quarantine destination names that could push the full quarantine path past the legacy Windows 260-character limit.

The quarantine filename is now shortened based on the quarantine directory length, not only a fixed flattened-name size. It still keeps the useful tail of the original path, and the manifest remains the source of truth for restore.

Closes #67.

Notes

• Keeps the filename under the 255-byte filename budget.
• Leaves room for collision suffixes such as .1 and the worst-case u32 suffix.
• Applies the full-path budget only on Windows; other platforms keep the filename-budget behavior.

Validation

• cargo fmt --check
• cargo test -p diskern-core -p diskern-cli --all-features
• cargo clippy -p diskern-core -p diskern-cli --all-targets --all-features -- -D warnings

[Muawiya-contact]

good