ci(docs): monitor Sent contract and documentation freshness

[sent-dm]

What changed

• Extended check_live_contract.py into the single fact-level OpenAPI and documentation monitor.
• Covered the llms index, OpenAPI, templates, webhooks, Sender Profiles, WABA, routing/RCS, and 10DLC sources.
• Added bounded per-source retries/timeouts and distinct drift, conflict, malformed, and unavailable outcomes.
• Preserved exits 1 for semantic drift/conflict and 2 for source/configuration failures.
• Added JSON diagnostics carrying expected value, observed value, source URL, and affected skills.
• Added offline simulations for endpoint removal, enum addition, property change, source conflict, timeout, and malformed response.
• Moved network checks into a dedicated scheduled/manual/release workflow, preserved the optional model-routing gate, and upload diagnostics with always().
• Kept PR validation fully offline and added freshness simulation coverage there.

Validation

• Live eight-source run: ok=8, no drift/conflict/malformed/unavailable results
• python3 scripts/generate_adapters.py --check
• python3 scripts/validate.py
• python3 scripts/test_fixtures.py
• python3 scripts/test_contracts.py
• python3 scripts/test_live_contract.py
• python3 scripts/test_validation_gates.py
• Python compilation and workflow YAML parsing
• Agent Skills validation for all canonical skills
• Strict Claude marketplace and plugin validation
• git diff --check

Stacked on the reference hygiene PR.