Add site runner setup doctor

[yoshifuminakamura]

Summary:

• add scripts/site/doctor_runner.sh for login-node static checks of a setup_runner.sh installation
• check generated helper scripts, runner config markers, Jacamar config markers, systemd unit shape, and persistent build-cache readiness without contacting GitLab or submitting scheduler jobs
• document the post-setup doctor command in add-site.md and add pytest coverage

Validation:

• bash -n scripts/site/doctor_runner.sh
• shellcheck -S error -f gcc scripts/site/doctor_runner.sh
• .venv/bin/python -m pytest result_server/tests/test_site_diagnostics.py result_server/tests/test_trigger_runner_setup_script.py result_server/tests/test_environment_snapshot_ci_scripts.py result_server/tests/test_site_runner_doctor_script.py -q
• .venv/bin/python result_server/tests/run_result_server_tests.py
• python3 scripts/tests/check_text_integrity.py
• python3 scripts/tests/check_app_contracts.py
• full shell bash -n and shellcheck -S error across scripts/, programs/, benchpark-bridge/scripts/