Make postpack cleanup cross-platform

[ryanduguid]

Summary

Replaces the POSIX-only rm -f postpack command with Node's cross-platform fs.rmSync(..., {force: true}).

The package already requires Node, so this adds no dependency and changes only lifecycle cleanup.

Verification at 295d131e1805baab3d457ab5342b651a87dc11f4

• npm ci;
• focused tests and build passed;
• npm pack --dry-run passed on Windows;
• oclif.manifest.json was removed after packing.

Compatibility with #38

A detached composition of #38 exact head 8a1120a1787125afc3eb1cb80f063373cafe4baa plus this commit produced c2ff3ba locally; no combined branch was pushed.

On that composition:

• clean npm ci audited 309 packages with zero vulnerabilities;
• all 130 tests and the build passed;
• npm pack --dry-run passed with 179 files (61.4 kB packed / 463.3 kB unpacked);
• postpack removed the generated manifest, and no tarball or untracked file remained.

By contrast, #38 alone reaches postpack and reproduces the unchanged rm failure on Windows. This confirms this focused portability change supplies the missing lifecycle fix and composes cleanly with the dependency refresh.