Add a reusable callable-dependency-review workflow that runs
actions/dependency-review-action with fail-on-severity: high, and
append a Sea Haven checklist to the PR template covering infra,
secrets, PITR, Slack, Confluence, memory, and cross-review.
Codifies the handbook PR description format (Summary/Validation/Tests/
Notes) as the default template GitHub prefills for every repo in the org
that lacks its own. Hidden hint comments carry the title, scope, and Jira
linking conventions so contributors fill in structure instead of a blank
box.