fix: clear Tutor Quality Stage 1 Sonar debt

[ttbombadil]

Scope

This maintenance PR fixes exactly the four pre-existing SonarQube New-Code findings that remained outside Tutor Quality Stage 2A:

• S7735 and S4624 in scripts/validate-tutor-course-content.mjs
• S4325 in server/services/course-content/tutor-quality-directory-validator.ts
• S3776 in server/services/course-content/tutor-quality-validator.ts

The changes are minimal and behavior-neutral: CLI argument branching/template construction, removal of redundant type assertions, and extraction of named validation helpers without changing validation rules. Targeted regression tests cover the existing CLI usage status and unknown-topic validation behavior.

Boundaries

• No Sonar configuration or New-Code baseline changes
• No Tutor behavior, Course-Content semantics, or Stage-1 quality-rule changes
• No Tutor Quality Stage 2A files or commits
• No merge is requested in this PR

Verification

All checks were run under Node 24.20.0:

• focused Course-Content validator tests: passed
• npm run test:tutor-quality: 24 files, 182 tests passed
• npm run test:unit: 238 files, 2443 tests passed, 1 existing skip
• npm run check: passed
• npm run check:docs: passed
• git diff --check: passed
• local SonarQube Quality Gate: passed
• New-Code violations: 0
• New-Code security hotspots reviewed: 100%
• New-Code coverage: 82.1%
• New-Code duplication: 0.21168%

The pre-push hook repeated the unit and coverage/Sonar checks successfully.