朱子纯 / test2

Step 4/6 (npm test) blew up with 'Missing script: test' on a fresh
frontend that hasn't wired up a test runner yet. Guard with a
node-based check on package.json#scripts.test so the gate skips
gracefully (with a log line) instead of failing.

Lint/build still always run when frontend is tracked — only the
test step is conditional on having a runner configured.

Untracked WIP directories (e.g. unfinished frontend/) were triggering
HAS_FRONTEND=1 via bare `[ -d frontend ]`, causing the pre-push gate
to lint/build code that isn't part of the push. Require non-empty
`git ls-files -- <dir>` so detection follows tracked content, matching
the CLAUDE.md rule that the gate covers current module + completed
modules only.

让 git pre-push 钩子触发的子 shell 也能拿到正确的 JDK21 路径与 mysql-client 路径，避免在 PATH 短的环境下编译/连库失败。