ci: consolidate node-ci checks into one job to cut billed minutes

[domengabrovsek]

Why

GitHub bills every Actions job a 1-minute minimum, even for a sub-second check. The shared node-ci.yml ran lint / format / typecheck / test / build as five separate jobs (plus an internal gate), each re-running npm ci. So a check suite that does seconds of real work cost 5-6 billed minutes per run - multiplied across every spoke repo that uses this reusable (real-estate, tax-calculator, personal-api, personal-app, personal-website, shupak-bot, parketarstvo).

This was a top driver of the account blowing through its 2000 free minutes in days.

What changed

• Collapse the five check jobs into one checks job that runs npm ci once, then each check as a continue-on-error step.
• A final Verify step fails the job if any check failed - so every check still runs and shows individually in the log, only the wasted per-job install + minute-rounding is gone.
• Drop the redundant internal gate job (consumers already aggregate this reusable in their own Gate check).

Impact

• Jobs per run: 7 -> 2 (checks + optional actionlint).
• npm ci runs once instead of 5x.
• Propagates to all spoke repos automatically (they pin @main).
• Estimated ~70% cut on spoke-repo CI minutes.

Trade-off

Checks now run sequentially in one job rather than in parallel. Each is sub-second after a single install, so wall-clock is comparable. Per-check branch-protection status entries are lost, but these private repos are on the Free plan and cannot use required status checks anyway.