test: make missing taskbrief smoke check hermetic

[rogerchappel]

Summary

• expose node through a dedicated temporary bin directory
• assert taskbrief cannot resolve before exercising the failure path
• retain the documented missing-binary error assertion and mocked success coverage

Commits

• 6a7871f test: isolate missing taskbrief smoke check

Verification

• corepack pnpm install --frozen-lockfile
• corepack pnpm run release:check
• corepack pnpm run smoke:init
• git diff --check

The full release check passes on a host where both globally installed node and taskbrief resolve from /usr/local/bin; CI will verify the GitHub Actions environment.