Replace placeholder local CI scripts with real test gates

[jmcte]

Context

The repository has real test, coverage, build, and mutation wiring, but the local CI helper scripts still look like generic bootstrap placeholders:

• scripts/ci/run-fast-checks.sh
• scripts/ci/run-extended-validation.sh

Meanwhile .github/workflows/ci.yml runs the real Node gates directly with pnpm lint, pnpm test:coverage, and pnpm build, and .github/workflows/extended-validation.yml has a separate mutation-testing job.

Scope

• Replace the placeholder fast-check script with the project's real cheap local gate.
• Replace the placeholder extended-validation script with the intended deeper local validation path.
• Keep the scripts shell-safe for the self-hosted Synology public runner policy.
• Update or add tests that assert these scripts run meaningful checks.

Acceptance Criteria

• bash scripts/ci/run-fast-checks.sh runs the expected fast checks and fails on test/build/lint failures.
• bash scripts/ci/run-extended-validation.sh runs fast checks plus the intended extended checks.
• Existing workflow tests are updated so placeholder scripts cannot regress back in.
• Documentation or comments clarify whether mutation testing remains a separate CI job or is part of local extended validation.