chore(update): drop no-op checksum sed

[CheapFuck]

What

In towel update --apply, the checksum verification piped the .sha256 through a sed before sha256sum -c:

# sha256sum -c expects the filename column to match the local file; rewrite it.
sed "s/${tarball}$/${tarball}/" "$checksum" | /usr/bin/sha256sum -c - >/dev/null 2>&1

The substitution s/${tarball}$/${tarball}/ replaces the filename with itself, so it never changes anything. The comment claims it rewrites the filename column, but the code is a no-op.

Why it's safe to drop

The release checksum is produced in .releaserc.yml with:

sha256sum "towel-${nextRelease.version}.tar.gz" > "towel-${nextRelease.version}.tar.gz.sha256"

That writes a bare filename in the standard two-space format (<hash> towel-<ver>.tar.gz). The tarball is downloaded under that same bare name into $tmpdir, and verification runs after cd "$tmpdir", so sha256sum -c "$checksum" matches the local file directly — no rewriting needed.

Change

 (
 	cd "$tmpdir"
-	# sha256sum -c expects the filename column to match the local file; rewrite it.
-	sed "s/${tarball}$/${tarball}/" "$checksum" | /usr/bin/sha256sum -c - >/dev/null 2>&1
+	/usr/bin/sha256sum -c "$checksum" >/dev/null 2>&1
 ) || {

No behavior change; removes dead code and a misleading comment.

Verification

• sha256sum -c <file>.sha256 verifies a release-format checksum directly (confirmed locally).
• shellcheck -x and shfmt -d pass on install, towel, and all towel-* scripts.