Run advisory structural validation on main pushes

[referencefield]

Development-only wiring change; this does not start acceptance.

Changes:

• run the existing advisory structural validator on every push to canonical main, including normal Operational Memory writes
• keep pull-request and manual-dispatch validation
• keep validation non-blocking and advisory
• clarify that main-push validation is a post-write repository-integrity check, while immediate persistence claims still depend on write/readback verification
• declare the trigger contract in PROTOCOL.yaml
• align OPERATIONS.md with the new behavior

protocol_version remains unreleased and protocol_status remains development.