npm shim: refuse a download with no verifiable sidecar

The skill launcher and `impeccable install` both fail closed when a
release binary's `.sha256` sidecar cannot be fetched or carries no hash:
they refuse rather than cache an unverified binary. The npm shim did not.
It only compared when a hash was present, so a 404, an empty sidecar, or
a truncated one all wrote the payload straight into
`~/.impeccable/bin/<version>/` and exec'd it.

It now refuses in the same cases, with wording that matches the launcher,
and writes nothing until the hash matches, so a refusal leaves the cache
dir empty. IMPECCABLE_BIN and the optional-dependency lookup are
untouched: neither downloads.

tests/cli-shim.test.mjs runs the real shim against a throwaway HTTP
server and covers missing, empty, and mismatched sidecars, plus the
matching-sidecar and IMPECCABLE_BIN paths. The two refusal cases fail
against the old shim.

Co-Authored-By: Claude Code <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vau2X53xGTjjTCXWMVBoNY

scripts/test-suites.mjs

@@ -63,6 +63,7 @@ export const SUITES = {
         timeoutMs: 180000,
         files: [
           'tests/ci-test-plan.test.mjs',
+          'tests/cli-shim.test.mjs',
           'tests/github-sheriff.test.mjs',
           'tests/hook-build.test.mjs',
           'tests/openai-plugin.test.mjs',