[patch] Reconcile managed release claims - #26

Summary

• add the August 29 managed-platform candidate gate to the canonical release record
• qualify GitHub, Slack, managed-platform, and returning-customer docs against that gate
• remove the live raw Slack install link until the authenticated deployed path is released
• validate the claims boundary in docs-check

Validation

• python3 scripts/check-task-agent-catalog.py
• python3 scripts/check-managed-release-claims.py
• Mintlify 4.2.687 build validation (Node 22 container)
• Mintlify 4.2.687 broken-link/anchor/redirect/snippet validation (Node 22 container)
• git diff --check