Prove entrypoint freshness sweep labels and clean state

[marcusglee11]

Summary

• adds a read-only GitHub label preflight for the doc-entrypoint freshness sweep
• validates labels before issue-creating upsert runs, so missing labels fail before FindingsDB/GitHub mutation
• proves quiet clean-state dry-run behavior: no findings, no rows, no receipt/no Telegram path

Control context

• LifeOS#125
• Operational-bus#325 queue item 2

Verification

• python3 -m pytest tests_doc/test_freshness_validator.py tests_doc/test_entrypoint_freshness_sweep.py -q → 41 passed
• python3 -m doc_steward.cli entrypoint-freshness-sweep . --dry-run --json → 0 findings, rows [], receipt null
• python3 -m doc_steward.cli entrypoint-freshness-sweep . --validate-labels --json → labels valid for sweep:inventory-hygiene, severity:warning
• python3 scripts/workflow/quality_gate.py check --scope changed --json → passed blocking checks; mypy advisory unavailable locally

Boundaries

No docs were edited, no issue was created, no scheduler/cron/runtime activation, no recurring sweep activation.