refactor: parallel lint/test jobs and setup-node-workspace composite action

[maxnoller]

Summary

• Splits the monolithic unit-tests job into parallel lint and test jobs — lint is fast and should not block on the slow test suite
• Adds .github/actions/setup-node-workspace composite action encapsulating the 3-step Node setup (setup-node + npm ci + npm run setup), preventing the container dep install from being missed in future jobs
• Aligns publish-release.yml publish-npm job to Node 22 with npm cache (absorbs chore: use npm ci in CI and align Node.js version across workflows #300)
• Uses npm ci throughout CI for lockfile-exact reproducible installs

Job graph after this PR

docker-preflight/agent   ─┐
docker-preflight/gateway  ─┤  all parallel
lint                      ─┤
test                      ─┤
  └─ npm-e2e ─────────────┘

Test plan

• CI passes on this branch
• lint and test jobs run in parallel (visible in Actions UI)
• npm-e2e waits for test before starting
• docker-preflight jobs use the composite action correctly
• No reference to old unit-tests job remains