fix: migration docs

[EmmaLouise2018]

Summary

• Add buying_mode as a required field on get_products to the breaking changes table
• Add comply-blocking warning callout for the buying_mode requirement
• Fix Accounts protocol row: protocol is required for all buyers — require_operator_auth determines which task to call (sync_accounts for implicit, list_accounts for explicit), not whether accounts are needed

Test plan

• Verify migration index page renders correctly with the new table row and warning callout
• Confirm links to get_products reference and v3 readiness checklist resolve properly
• Verify Accounts protocol row accurately reflects the docs at /docs/accounts/overview