Follow up on semver-checks target-directory tradeoffs

Follow-up work from PR #702:

- Document that package-specific relocation can split one release's baseline artifacts between `\target` and `\oxi-sc`, including the resulting cleanup behavior.
- Document or mitigate the trust boundary introduced by a deterministic target directory at the volume root, where another local user could pre-create the directory and control its ACL.
- Update the operator-facing help in `scripts/release-packages.ps1` to describe conditional `CARGO_TARGET_DIR` relocation, explicit-variable precedence, and manual cleanup of `\oxi-sc`.
- Guard the result of `GetPathRoot` before calling `StartsWith`, preserving the fail-open behavior when a whitespace-only repository root produces `$null`.

The upstream `cargo-semver-checks` feature request is tracked separately in obi1kenobi/cargo-semver-checks#1720.