Improve doctor.ts mutation coverage

[jmcte]

Context

Stryker mutation testing is configured and currently passes the permissive break threshold, but src/lib/doctor.ts is the weakest configured target:

doctor.ts mutation score: 54.09%
doctor.ts covered mutation score: 59.12%

That leaves too many operator-facing health-check branches alive after mutation.

Scope

• Review surviving and no-coverage mutants for src/lib/doctor.ts.
• Add focused tests for missing environment, missing host fields, GitHub verification failure surfaces, Lume status artifact handling, and user-facing report details where mutants survived.
• Prefer behavioral assertions over broad snapshots.
• Keep tests local and fully mocked; do not require live Synology or GitHub credentials.

Acceptance Criteria

• pnpm test -- test/doctor.test.ts passes.
• pnpm mutation-test shows a materially improved doctor.ts score.
• Surviving mutants that remain are documented as low-value formatting/string mutants or intentionally deferred.
• No live external services are required by the new tests.