refactor: remove obsolete exported dead code

[ttbombadil]

Summary

Removes the ten pre-existing Knip diagnostics from the repository instead of
carrying them permanently as accepted baseline debt.

The cleanup removes only unused exports/types and two obsolete deprecated
internal compatibility aliases.

Removed diagnostics

Unused exports:

• anchorCourseContentRevision
• isTutorPlanningTransition
• DockerCompileSemaphore
• getDockerCompileSemaphore

Unused exported types:

• CourseContentManifest
• TutorPhase
• TutorQualityCases
• TopicExtension

The two duplicate-export diagnostics disappear with the obsolete Sandbox
compatibility aliases.

Compatibility review

The removed symbols have no consumers in repository code, tests or
documentation.

The two Sandbox aliases were marked as deprecated internal compatibility
aliases. No supported public package surface or documented external use was
found.

Used schemas and parser logic remain intact; in particular
courseContentManifestSchema is unchanged.

Verification

• npx knip — 0 findings, exit 0
• npm run check — passed
• npm run check:docs — passed
• npm run test:tutor-quality — 216 passed
• npm run test:unit — 2477 passed / 1 skipped
• git diff --check — passed
• bash -n run-tests.sh — passed
• full ./run-tests.sh — passed
• SonarQube Quality Gate — passed
• Sonar violations — 0

Scope

No changes to:

• run-tests.sh
• Knip configuration
• baseline configuration
• Tutor Quality Stage 2B
• PR feat(tutor-quality): add stage 2b semantic foundation #118
• PR chore(quality): make knip gate baseline-aware #119
• provider behavior