fix: restore no-token repository validation

[vultuk]

Summary

• restore tokenless repository existence validation with a manual GitHub HEAD probe
• preserve authenticated API validation, ignore stale unauthenticated misses when a token is present, and only cache definitive anonymous results
• add page-view regressions for positive/negative caching, redirect handling, transient probe failures, and authenticated fallback

Validation

• npx bun run typecheck
• npx bun test test/repository-service-page-view.test.ts
• npx bun test test/repo-api-route.test.ts

Review

• pre-commit review gate: reviewer-only fallback via codex exec using the loaded review-changes reviewer contract because full orchestration was not available in this cron-run environment

Closes #42