Removes auto-deploy-on-PR-merge trigger and the old PR-merge gate. Deploy now
triggers only via workflow_dispatch, with chart_ref required (no default) as
the literal sientia-helm-chart branch bumped and deployed — hard error if it
doesn't exist, no silent fallback to development/main.
- .github/workflows/deploy.yml: drop pull_request trigger, add chart_ref
input, branch_name now github.ref_name, drop pull-requests:read permission
- Delete .github/workflows/require-chart-ref-label.yml
- README.md: update Deploy section to describe manual trigger
[skip ci]